Copyright 1998 by the Educational Publishing Foundation
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Review of General Psychology
1998, Vol. 2, No. 2, 175-220

Confirmation Bias: A Ubiquitous Phenomenon in Many Guises
Raymond S. Nickerson

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Tufts University
Confirmation bias, as the term is typically used in the psychological literature, connotes
the seeking or interpreting of evidence in ways that are partial to existing beliefs,
expectations, or a hypothesis in hand. The author reviews evidence of such a bias in avariety of guises and gives examples of its operation in several practical contexts.
Possible explanations are considered, and the question of its utility or disutility is
discussed.

When men wish to construct or support a theory, how
they torture facts into their service! (Mackay, 1852/
1932, p. 552)
Confirmation bias is perhaps the best known and mostwidely accepted notion of inferential error to come outof the literature on human reasoning. (Evans, 1989, p. 41)

If one were to attempt to identify a single
problematic aspect of human reasoning thatdeserves attention above all others, the confirmation bias would have to be among the candidates
for consideration. Many have written about thisbias, and it appears to be sufficiently strong andpervasive that one is led to wonder whether thebias, by itself, might account for a significant
fraction of the disputes, altercations, and misunderstandings that occur among individuals,
groups, and nations.
Confirmation bias has been used in thepsychological literature to refer to a variety ofphenomena. Here I take the term to represent a
generic concept that subsumes several more
specific ideas that connote the inappropriate
bolstering of hypotheses or beliefs whose truth
is in question.
Deliberate Versus Spontaneous
Case Building
There is an obvious difference between
impartially evaluating evidence in order to cometo an unbiased conclusion and building a case tojustify a conclusion already drawn. In the first
instance one seeks evidence on all sides of a
Correspondence concerning this article should be addressed to Raymond S. Nickerson, Department of Psychology, Paige Hall, Tufts University, Medford, Massachusetts
02155. Electronic mail may be sent to mickerson@infonet.
tufts.edu.

question, evaluates it as objectively as one can,and draws the conclusion that the evidence, in
the aggregate, seems to dictate. In the second,
one selectively gathers, or gives undue weight
to, evidence that supports one's position whileneglecting to gather, or discounting, evidence
that would tell against it.
There is a perhaps less obvious, but also
important, difference between building a caseconsciously and deliberately and engaging incase-building without being aware of doing so.
The first type of case-building is illustrated bywhat attorneys and debaters do. An attorney's
job is to make a case for one or the other side ofa legal dispute. The prosecutor tries to marshal
evidence to support the contention that a crime
has been committed; the defense attorney tries
to present evidence that will support thepresumption that the defendant is innocent.
Neither is committed to an unbiased weighing of
all the evidence at hand, but each is motivated toconfirm a particular position. Debaters also
would be expected to give primary attention toarguments that support the positions they are
defending; they might present counterarguments, but would do so only for the purpose ofpointing out their weaknesses.
As the term is used in this article and, I
believe, generally by psychologists, confirmation bias connotes a less explicit, less consciously one-sided case-building process. It
refers usually to unwitting selectivity in theacquisition and use of evidence. The line
between deliberate selectivity in the use ofevidence and unwitting molding of facts to fit
hypotheses or beliefs is a difficult one to draw inpractice, but the distinction is meaningfulconceptually, and confirmation bias has more to
do with the latter than with the former. The
175

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176

RAYMOND S. NICKERSON

assumption that people can and do engage incase-building unwittingly, without intending totreat evidence in a biased way or even being
aware of doing so, is fundamental to theconcept.
The question of what constitutes confirmationof a hypothesis has been a controversial matter
among philosophers and logicians for a longtime (Salmon, 1973). The controversy is exemplified by Hempel's (1945) famous argumentthat the observation of a white shoe isconfirmatory for the hypothesis "All ravens areblack," which can equally well be expressed incontrapositive form as "All nonblack things arenonravens." Goodman's (1966) claim that
evidence that something is green is equally goodevidence that it is "grue"—grue being definedas green before a specified future date and blue
thereafter—also provides an example. A large
literature has grown up around these and similar
puzzles and paradoxes. Here this controversy islargely ignored. It is sufficiently clear for the
purposes of this discussion that, as used ineveryday language, confirmation connotes evidence that is perceived to support—to increase
the credibility of—a hypothesis.
I also make a distinction between what might
be called motivated and unmotivated forms ofconfirmation bias. People may treat evidence ina biased way when they are motivated by thedesire to defend beliefs that they wish to
maintain. (As already noted, this is not tosuggest intentional mistreatment of evidence;
one may be selective in seeking or interpreting
evidence that pertains to a belief without being
deliberately so, or even necessarily being awareof the selectivity.) But people also may proceed
in a biased fashion even in the testing of
hypotheses or claims in which they have nomaterial stake or obvious personal interest. The
former case is easier to understand in commonsense terms than the latter because one canappreciate the tendency to treat evidence
selectively when a valued belief is at risk. But it
is less apparent why people should be partial intheir uses of evidence when they are indifferent
to the answer to a question in hand. An adequate
account of the confirmation bias must encompass both cases because the existence of each iswell documented.
There are, of course, instances of one wishing
to disconfirm a particular hypothesis. If, forexample, one believes a hypothesis to be untrue,

one may seek evidence of that fact or give undue
weight to such evidence. But in such cases, thehypothesis in question is someone else's belief.
For the individual who seeks to disconfirm sucha hypothesis, a confirmation bias would be abias to confirm the individual's own belief,
namely that the hypothesis in question is false.

A Long-Recognized Phenomenon
Motivated confirmation bias has long been
believed by philosophers to be an important
determinant of thought and behavior. Francis
Bacon (1620/1939) had this to say about it, forexample:The human understanding when it has once adopted an
opinion (either as being the received opinion or as
being agreeable to itself) draws all things else tosupport and agree with it. And though there be a greaternumber and weight of instances to be found on theother side, yet these it either neglects and despises, orelse by some distinction sets aside and rejects; in orderthat by this great and pernicious predetermination theauthority of its former conclusions may remain
inviolate.. . . And such is the way of all superstitions,
whether in astrology, dreams, omens, divine judgments, or the like; wherein men, having a delight insuch vanities, mark the events where they are fulfilled,
but where they fail, although this happened muchoftener, neglect and pass them by. (p. 36)

Bacon noted that philosophy and the sciences
do not escape this tendency.
The idea that people are prone to treat
evidence in biased ways if the issue in question
matters to them is an old one among psychologists also:
If we have nothing personally at stake in a dispute
between people who are strangers to us, we areremarkably intelligent about weighing the evidence andin reaching a rational conclusion. We can be convinced
in favor of either of the fighting parties on the basis ofgood evidence. But let the fight be our own, or let ourown friends, relatives, fraternity brothers, be parties tothe fight, and we lose our ability to see any other side of
the issue than our own. .. . The more urgent theimpulse, or the closer it comes to the maintenance of
our own selves, the more difficult it becomes to berational and intelligent. (Thurstone, 1924, p. 101)

The data that I consider in what follows do
not challenge either the notion that people
generally like to avoid personally disquieting
information or the belief that the strength of abias in the interpretation of evidence increases
with the degree to which the evidence relates
directly to a dispute in which one has a personal

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CONFIRMATION BIAS

stake. They are difficult to reconcile, however,with the view that evidence is treated in a totally
unbiased way if only one has no personal
interest in that to which it pertains.
The following discussion of this widely
recognized bias is organized in four major
sections. In the first, I review experimental
evidence of the operation of a confirmation bias.
In the second, I provide examples of the bias atwork in practical situations. The third section
notes possible theoretical explanations of thebias that various researchers have proposed. The
fourth addresses the question of the effects ofthe confirmation bias and whether it serves any
useful purposes.
Experimental Studies
A great deal of empirical evidence supports
the idea that the confirmation bias is extensive
and strong and that it appears in many guises.
The evidence also supports the view that once
one has taken a position on an issue, one's
primary purpose becomes that of defending or
justifying that position. This is to say thatregardless of whether one's treatment of evidence was evenhanded before the stand was
taken, it can become highly biased afterward.

Hypothesis-Determined Information
Seeking and Interpretation
People tend to seek information that they
consider supportive of favored hypotheses orexisting beliefs and to interpret information in
ways that are partial to those hypotheses or
beliefs. Conversely, they tend not to seek andperhaps even to avoid information that would beconsidered counterindicative with respect tothose hypotheses or beliefs and supportive ofalternative possibilities (Koriat, Lichtenstein, &Fischhoff, 1980).
Beyond seeking information that is supportive of an existing hypothesis or belief, it appears
that people often tend to seek only, or primarily,
information that will support that hypothesis or
belief in a particular way. This qualification isnecessary because it is generally found thatpeople seek a specific type of information thatthey would expect to find, assuming the
hypothesis is true. Also, they sometimes appear
to give weight to information that is consistent
with a hypothesis but not diagnostic with respect177

to it. These generalizations are illustrated byseveral of the following experimental findings.
Restriction of attention to a favored hypothesis. If one entertains only a single possible
explanation of some event or phenomenon, one
precludes the possibility of interpreting data as
supportive of any alternative explanation. Even
if one recognizes the possibility of otherhypotheses or beliefs, perhaps being aware that
other people hold them, but is strongly committed to a particular position, one may fail toconsider the relevance of information to thealternative positions and apply it (favorably)
only to one's own hypothesis or belief.
Restricting attention to a single hypothesis
and failing to give appropriate consideration toalternative hypotheses is, in the Bayesianframework, tantamount to failing to take likelihood ratios into account. The likelihood ratio is
the ratio of two conditional probabilities,p(D\Hx)lp(p\Hj), and represents the probabilityof a particular observation (or datum) if one
hypothesis is true relative to the probability ofthat same observation if the other hypothesis istrue. Typically there are several plausible
hypotheses to account for a specific observation,
so a given hypothesis would have several
likelihood ratios. The likelihood ratio for ahypothesis and its complement, p{D\H)lp{D\~H), is of special interest, however,
because an observation gives one little evidence
about the probability of the truth of a hypothesis
unless the probability of that observation, giventhat the hypothesis is true, is either substantiallylarger or substantially smaller than the probability of that observation, given that the hypothesis
is false.
The notion of diagnosticity reflects theimportance of considering the probability of anobservation conditional on hypotheses otherthan the favored one. An observation is said tobe diagnostic with respect to a particularhypothesis to the extent that it is consistent withthat hypothesis and not consistent, or not as
consistent, with competing hypotheses and inparticular with the complementary hypothesis.
One would consider an observation diagnostic
with respect to a hypothesis and its complement
to the degree that the likelihood ratio, p{D\H)lp(D\~H), differed from 1. An observation thatis consistent with more than one hypothesis issaid not to be diagnostic with respect to thosehypotheses; when one considers the probability

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178

RAYMOND S. NICKERSON

of an observation conditional on only a single
hypothesis, one has no way of determining
whether the observation is diagnostic.
Evidence suggests that people often do theequivalent of considering only p(D\H) andfailing to take into account the ratio of this andp(D\~H), despite the fact that considering only
one of these probabilities does not provide a
legitimate basis for assessing the credibility of//
(Beyth-Marom & Fischhoff, 1983; Doherty &Mynatt, 1986; Doherty, Mynatt, Tweney, &Schiavo, 1979; Griffin & Tversky, 1992; Kern &Doherty, 1982; Troutman & Shanteau, 1977).
This tendency to focus exclusively on the casein which the hypothesis is assumed to be true isoften referred to as a tendency toward pseudodiagnosticity (Doherty & Mynatt, 1986; Doherty
et al., 1979; Fischhoff & Beyth-Marom, 1983;
Kern & Doherty, 1982). Fischhoff and BeythMarom (1983) have argued that much of what
has been interpreted as a confirmation bias canbe attributed to such a focus and the consequential failure to consider likelihood ratios.
Preferential treatment of evidence supporting
existing beliefs. Closely related to the restriction of attention to a favored hypothesis is thetendency to give greater weight to informationthat is supportive of existing beliefs or opinions
than to information that runs counter to them.
This does not necessarily mean completely
ignoring the counterindicative information but
means being less receptive to it than tosupportive information—more likely, for example, to seek to discredit it or to explain it
away.
Preferential treatment of evidence supporting
existing beliefs or opinions is seen in thetendency of people to recall or produce reasonssupporting the side they favor—my-sidebias—on a controversial issue and not to recall
or produce reasons supporting the other side
(Baron, 1991, 1995; Perkins, Allen, & Hafner,
1983; Perkins, Farady, & Bushey, 1991). It
could be either that how well people remember areason depends on whether it supports theirposition, or that people hold a position because
they can think of more reasons to support it.
Participants in the study by Perkins, Farady, andBushey were capable of generating reasons forholding a view counter to their own when
explicitly asked to do so; this finding led
Tishman, Jay, and Perkins (1993) to interpret the

failure to do so spontaneously as a motivational
problem as distinct from a cognitive limitation.
Baron (1995) found that, when asked to judge
the quality of arguments, many people werelikely to rate one-sided arguments higher than
two-sided arguments, suggesting that the bias is
at least partially due to common beliefs about
what makes an argument strong. In keeping withthis result, participants in a mock jury trial who
tended to use evidence selectively to build one
view of what happened expressed greater
confidence in their decisions than did those who
spontaneously tried to weigh both sides of thecase (D. Kuhn, Weinstock, & Flaton, 1994).
When children and young adults were given
evidence that was inconsistent with a theory
they favored, they often "either failed toacknowledge discrepant evidence or attended to
it in a selective, distorting manner. Identicalevidence was interpreted one way in relation to afavored theory and another way in relation to atheory that was not favored" (D. Kuhn, 1989, p.
677). Some of Kuhn's participants were unable
to indicate what evidence would be inconsistent
with their theories; some were able to generatealternative theories when asked, but they did not
do so spontaneously. When they were asked torecall their theories and the related evidence that
had been presented, participants were likely torecall the evidence as being more consistent
with the theories than it actually was. The
greater perceived consistency was achieved
sometimes by inaccurate recall of theory andsometimes by inaccurate recall of evidence.
Looking only or primarily for positive cases.
What is considerably more surprising than thefact that people seek and interpret information in
ways that increase their confidence in favoredhypotheses and established beliefs is the factthat they appear to seek confirmatory information even for hypotheses in whose truth value
they have no vested interest. In their pioneering
concept-discovery experiments, Bruner, Goodnow, and Austin (1956) found that participants
often tested a hypothesized concept by choosing
only examples that would be classified as
instances of the sought-for concept if the
hypothesis were correct. This strategy precludes
discovery, in some cases, that an incorrect
hypothesis is incorrect. For example, suppose
the concept to be discovered is small circle andone's hypothesis is small red circle. If one tests
the hypothesis by selecting only things that areThis document is copyrighted by the American Psychological Association or one of its allied publishers.
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CONFIRMATION BIAS

small, red, and circular, one will never discoverthat the class denned by the concept includes
also small circular things that are yellow or blue.
Several investigators (Levine, 1970; Millward& Spoehr, 1973; Taplin, 1975; Tweney et al.,
1980; Wason & Johnson-Laird, 1972) subsequently observed the same behavior of participants testing only cases that are members of the
hypothesized category.
Some studies demonstrating selective testing
behavior of this sort involved a task invented by
Wason (1960) in which people were asked tofind the rule that was used to generate specified
triplets of numbers. The experimenter presented
a triplet, and the participant hypothesized therule that produced it. The participant then tested
the hypothesis by suggesting additional triplets
and being told, in each case, whether it wasconsistent with the rule to be discovered. People
typically tested hypothesized rules by producing
only triplets that were consistent with them.
Because in most cases they did not generate any
test items that were inconsistent with the
hypothesized rule, they precluded themselvesfrom discovering that it was incorrect if thetriplets it prescribed constituted a subset of thoseprescribed by the actual rule. Given the triplet
2-4-6, for example, people were likely to comeup with the hypothesis successive even numbers
and then proceed to test this hypothesis bygenerating additional sets of successive evennumbers. If the 2-4-6 set had actually been
produced by the rule numbers increasing by 2,
numbers increasing in size, or any three positive
numbers, the strategy of using only sets ofsuccessive even numbers would not reveal theincorrectness of the hypothesis because every
test item would get a positive response.
The use only of test cases that will yield apositive response if a hypothesis under consideration is correct not only precludes discovering
the incorrectness of certain types of hypotheseswith a correct hypothesis, this strategy would
not yield as strongly confirmatory evidence,
logically, as would that of deliberately selecting
tests that would show the hypothesis to be
wrong, if it is wrong, and failing in the attempt.
To the extent that the strategy of looking only
for positive cases is motivated by a wish to findconfirmatory evidence, it is misguided. The
results this endeavor will yield will, at best, beconsistent with the hypothesis, but the confirmatory evidence they provide will not be as

179

compelling as would the failure of a rigorous
attempt at disconfirmation.
This point is worth emphasizing because thepsychological literature contains many references to the confirmatory feedback a participant
gets when testing a hypothesis with a positivecase. These references do not generally distinguish between confirmatory in a logical senseand confirmatory in a psychological sense. The
results obtained by Wason (1960) and otherssuggest that feedback that is typically interpreted by participants to be strongly confirmatory often is not logically confirmatory, or atleast not strongly so. The "confirmation" theparticipant receives in this situation is, to somedegree, illusory. This same observation applies
to other studies mentioned in the remainder ofthis article.
In an early commentary on the triplet-ruletask, Wetherick (1962) argued that the experimental situation did not reveal the participants'
intentions in designating any particular triplet as
a test case. He noted that any test triplet could
either conform or not conform to the rule, as
defined by the experimenter, and it could alsoeither conform or not conform to the hypothesis
being considered by the participant. Any given
test case could relate to the rule and hypothesisin combination in any of four ways: conformconform, conform-not conform, not conformconform, and not conform-not conform. Wetherick argued that one could not determine
whether an individual was intentionally attempting to eliminate a candidate hypothesis unless
one could distinguish between test cases thatwere selected because they conformed to ahypothesis under consideration and those thatwere selected because they did not.
Suppose a participant selects the triplet 3-5-7
and is told that it is consistent with the rule (therule being any three numbers in ascendingorder). The participant might have chosen this
triplet because it conforms to the hypothesis
being considered, say numbers increasing by
two, and might have taken the positive response
as evidence that the hypothesis is correct. On theother hand, the participant could have selected
this triplet in order to eliminate one or more
possible hypotheses (e.g., even numbers ascending; a number, twice the number, three times thenumber). In this case, the experimenter's
positive response would constitute the discon-

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180

RAYMOND S. NICKERSON

firming evidence (with respect to these hypoth- Zuckerman, Knee, Hodgins, & Miyake, 1995).
eses) the participant sought.
Fischhoff and Beyth-Marom (1983) also noted
Wetherick (1962) also pointed out that a test the possibility that participants in such experitriplet may logically rule out possible hypoth- ments tend to assume that the hypothesis theyeses without people being aware of the fact are asked to test is true and select questions that
because they never considered those hypoth- would be the least awkward to answer if that iseses. A positive answer to the triplet 3-5-7 the case. For instance, participants asked
logically eliminates even numbers ascending assumed extroverts (or introverts) questions thatand a number, twice the number, three times the extroverts (or introverts) would find particularly
number, among other possibilities, regardless of easy to answer.
whether the participant thought of them. But of
Overweighting positive confirmatory incourse, only if the triplet was selected with the stances. Studies of social judgment provide
intention of ruling out those options should its evidence that people tend to overweight positive
selection be taken as an instance of a falsifica- confirmatory evidence or underweight negativetion strategy. Wetherick's point was that without discomfirmatory evidence. Pyszczynski andknowing what people have in mind in making Greenberg (1987) interpreted such evidence as
the selections they do, one cannot tell whether supportive of the view that people generally
they are attempting to eliminate candidates from require less hypothesis-consistent evidence tofurther consideration or not.
accept a hypothesis than hypothesis-inconsistent
Wason (1962, 1968/1977) responded to this information to reject a hypothesis. These
objection with further analyses of the data from investigators argued, however, that this asymmethe original experiment and data from additional try is modulated by such factors as the degree ofexperiments showing that although some partici- confidence one has in the hypothesis to begin
pants gave evidence of understanding the with and the importance attached to drawing a
concept of falsification, many did not. Wason correct conclusion. Although they saw the needsummarized the findings from these experi- for accuracy as one important determinant ofments this way: "there would appear to be hypothesis-evaluating behavior, they suggested
compelling evidence to indicate that even that other motivational factors, such as needs forintelligent individuals adhere to their own self-esteem, control, and cognitive consistency,
hypotheses with remarkable tenacity when they also play significant roles.
can produce confirming evidence for them"People can exploit others' tendency to over(1968/1977, p. 313).
weight (psychologically) confirming evidence
In other experiments in which participants and underweight disconfirming evidence forhave been asked to determine which of several many purposes. When the mind reader, forhypotheses is the correct one to explain some example, describes one's character in more-orsituation or event, they have tended to ask less universally valid terms, individuals whoquestions for which the correct answer would be want to believe that their minds are being readyes if the hypothesis under consideration were will have little difficulty finding substantiatingtrue (Mynatt, Doherty, & Tweney, 1977; Shak- evidence in what the mind reader says if theylee & Fischhoff, 1982). These experiments are focus on what fits and discount what does notamong many that have been taken to reveal not and if they fail to consider the possibility that
only a disinclination to test a hypothesis by equally accurate descriptions can be produced if
selecting tests that would show it to be false if it their minds are not being read (Fischhoff &is false, but also a preference for questions that Beyth-Marom, 1983; Forer, 1949; Hyman,
will yield a positive answer if the hypothesis is 1977). People who wish to believe in astrology
or the predictive power of psychics will have notrue.
Others have noted the tendency to ask problem finding some predictions that havequestions for which the answer is yes if the turned out to be true, and this may suffice tohypothesis being tested is correct in the context strengthen their belief if they fail to considerof experiments on personality perception (Hod- either predictions that proved not to be accurate
gins & Zuckerman, 1993; Schwartz, 1982; or the possibility that people without the ability
Strohmer & Newman, 1983; Trope & Bassock, to see the future could make predictions with
1982, 1983; Trope, Bassock, & Alon, 1984; equally high (or low) hit rates. A confirmation

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CONFIRMATION BIAS

bias can work here in two ways: (a) people may
attend selectively to what is said that turns out tobe true, ignoring or discounting what turns out
to be false, and (b) they may consider onlyp(D\H), the probability that what was said
would be said if the seer could really see, andfail to consider p(D\~H), the probability thatwhat was said would be said if the seer had no
special psychic powers. The tendency of gamblers to explain away their losses, thus permitting themselves to believe that their chances of
winning are higher than they really are, also
illustrates the overweighting of supportive
evidence and the underweighting of opposing
evidence (Gilovich, 1983).
Seeing what one is looking for. People
sometimes see in data the patterns for which
they are looking, regardless of whether thepatterns are really there. An early study by
Kelley (1950) of the effect of expectations onsocial perception found that students' perceptions of social qualities (e.g., relative sociability,
friendliness) of a guest lecturer were influenced
by what they had been led to expect from a priordescription of the individual. Forer (1949)
demonstrated the ease with which people could
be convinced that a personality sketch was an
accurate depiction of themselves and theirdisinclination to consider how adequately thesketch might describe others as well.
Several studies by Snyder and his colleagues
involving the judgment of personality traits lend
credence to the idea that the degree to which
what people see or remember corresponds towhat they are looking for exceeds the correspondence as objectively assessed (Snyder, 1981,
1984; Snyder & Campbell, 1980; Snyder &Gangestad, 1981; Snyder & Swann, 1978a,1978b). In a study representative of this body ofwork, participants would be asked to assess the
personality of a person they are about to meet.
Some would be given a sketch of an extrovert(sociable, talkative, outgoing) and asked todetermine whether the person is that type.
Others would be asked to determine whether the
person is an introvert (shy, timid, quiet).
Participants tend to ask questions that, if given
positive answers, would be seen as strongly
confirmatory and that, if given negative answers, would be weakly disconfirmatory of thepersonality type for which they are primed tolook (Snyder, 1981; Snyder & Swann, 1978a).
Some of the questions that people ask in these181

situations are likely to evoke similar answers
from extroverts and introverts (Swann, Giuliano, & Wegner, 1982)—answers not very
diagnostic with respect to personality type.
When this is the case, askers find it easy to see
the answers they get as supportive of the
hypothesis with which they are working, independently of what that hypothesis is.
The interpretation of the results of thesestudies is somewhat complicated by the findingof a tendency for people to respond to questions
in a way that, in effect, acquiesces to whatever
hypothesis the interrogator is entertaining (Lenski & Leggett, 1960; Ray, 1983; Schuman &Presser, 1981; Snyder, 1981; Zuckerman et al.,1995). For example, if the interrogator hypothesizes that the interviewee is an extrovert andasks questions that an extrovert would be
expected to answer in the affirmative, theinterviewee is more likely than not to answer in
the affirmative independently of his or her
personality type. Snyder, Tanke, and Berscheid(1977) have reported a related phenomenon.
They found that male participants acted differently in a phone conversation toward female
participants who had been described as attractive than toward those who had been described
as unnattractive, and that their behavior evoked
more desirable responses from the "attractive"
than from the "unattractive" partners.
Such results suggest that responders may
inadvertently provide evidence for a working
hypothesis by, in effect, accepting the assumption on which the questions or behavior are
based and behaving in a way that is consistent
with that assumption. Thus the observer's
expectations become self-fulfilling prophecies
in the sense suggested by Merton (1948, 1957);
the expectations are "confirmed" because thebehavior of the observed person has been
shaped by them, to some degree. Studies in
education have explored the effect of teachers'
expectations on students' performance and haveobtained similar results (Dusek, 1975; Meichenbaum, Bowers, & Ross, 1969; Rosenthal, 1974;
Rosenthal & Jacobson, 1968; Wilkins, 1977;
Zanna, Sheras, Cooper, & Shaw, 1975).
Darley and Fazio (1980) noted the importanceof distinguishing between the case in which the
behavior of a target person has changed in
response to the perceiver's expectation-guided
actions and that in which behavior has notchanged but is interpreted to have done so in aThis document is copyrighted by the American Psychological Association or one of its allied publishers.
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182

RAYMOND S. NICKERSON

way that is consistent with the perceiver's
expectations. Only the former is considered an
example of a self-fulfilling prophecy. In thatcase, the change in behavior should be apparent
to an observer whose observations are notsubject to distortion by the expectations inquestion. When people's actions are interpreted
in ways consistent with observers' expectationsin the absence of any interaction between
observer and observed, the self-fulfillingprophecy effect cannot be a factor.
Darley and Gross (1983) provided evidenceof seeing what one is looking for under the latterconditions noted above. These authors had twogroups of people view the same videotape of achild taking an academic test. One of the groups
had been led to believe that the child's
socioeconomic background was high and theother had been led to believe that it was low. The
former group rated the academic abilities, as
indicated by what they could see of her
performance on the test, as above grade level,
whereas the latter group rated the same performance as below grade level. Darley and Grosssaw this result as indicating that the participantsin their study formed a hypothesis about thechild's abilities on the basis of assumptions
about the relationship between socioeconomic
status and academic ability and then interpreted
what they saw in the videotape so as to make it
consistent with that hypothesis.
Numerous studies have reported evidence ofparticipants seeing or remembering behavior
that they expect. Sometimes the effects occur
under conditions in which observers interact
with the people observed and sometimes underconditions in which they do not. "Confirmed"
expectations may be based on ethnic (Duncan,1976), clinical (Langer & Abelson, 1974;
Rosenhan, 1973; Swann et al., 1982), educational (Foster, Schmidt, & Sabatino, 1976;
Rosenthal & Jacobson, 1968), socioeconomic(Rist, 1970), and lifestyle (Snyder & Uranowitz,1978) stereotypes, among other factors. And
they can involve induced expectations regarding
oneself as well as others (Mischel, Ebbensen, &Zeiss, 1973).
Pennebaker and Skelton (1978) have pointed
out how a confirmation bias can reinforce theworst fears of a hypochondriac. The body moreor less continuously provides one with anassortment of signals that, if attended to, could

be interpreted as symptoms of illness of one sortor another. Normally people ignore thesesignals. However, if one suspects that one is ill,
one is likely to begin to attend to these signals
and to notice those that are consistent with theassumed illness. Ironically, the acquisition of
factual knowledge about diseases and their
symptoms may exacerbate the problem. Upon
learning of a specific illness and the symptoms
that signal its existence, one may look for those
symptoms in one's own body, thereby increasing the chances of detecting them even if they
are not out of the normal range (Woods,
Matterson, & Silverman, 1966).
Similar observations apply to paranoia and avariety of neurotic or psychotic states. If one
believes strongly that one is a target of otherpeople's ill will or aggression, one is likely to beable to fit many otherwise unaccounted-for
incidents into this view. As Beck (1976) has
suggested, the tendency of people sufferingfrom mental depression to focus selectively on
information that gives further reason to be
depressed and ignore information of a morepositive nature could help perpetuate the depressed state.
The results of experiments using such abstract
tasks as estimating the proportions of beads ofdifferent colors in a bag after observing thecolor(s) of one or a few beads shows that data
can be interpreted as favorable to a working
hypothesis even when the data convey nodiagnostic information. The drawing of beads ofa given color may increase one's confidence in ahypothesis about the color distribution in the
bag even when the probability of drawing a beadof that color is the same under the working
hypothesis and its complement (Pitz, 1969;
Troutman & Shanteau, 1977). After one hasformed a preference for one brand of acommercial product over another, receiving
information about an additional feature that iscommon to the two brands may strengthen one's
preexisting preference (Chernev, 1997). Similarly, observers of a sports event may describe itvery differently depending on which team theyfavor (Hastorf & Cantril, 1954).
It is true in science as it is elsewhere (Mitroff,1974) that what one sees—actually or metaphorically—depends, to no small extent, on what one
looks for and what one expects. Anatomist A.
Kolliker criticized Charles Darwin for having

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CONFIRMATION BIAS

advanced the cause of teleological thinking, andbotanist Asa Gray praised Darwin for the same
reason. Meanwhile, biologist T. H. Huxley andnaturalist Ernst Haeckel praised Darwin forthoroughly discrediting thinking of this kind
(Gigerenzer et al., 1989).
Several investigators have stressed the importance of people's expectations as sources of bias
in their judgments of covariation (Alloy &Abramson, 1980; Alloy & Tabachnik, 1984;
Camerer, 1988; Crocker, 1981; Golding &Rorer, 1972; Hamilton, 1979; D. Kuhn, 1989;
Nisbett & Ross, 1980). The belief that twovariables are related appears to increase thechances that one will find evidence consistent
with the relationship and decrease the chancesof obtaining evidence that tends to disconfirm it.
Judgments of covariation tend to be moreaccurate when people lack strong preconceptions of the relationship between the variables ofinterest or when the relationship is consistent
with their preconceptions than when they havepreconceptions that run counter to the relationship that exists.
The perception of a correlation where none
exists is sometimes referred to as an illusory
correlation. The term also has been applied
when the variables in question are correlated but
the correlation is perceived to be higher than it
actually is Chapman and Chapman (1967a,
1967b, 1969; see also Chapman, 1967) did early
work on the phenomenon. These investigators
had participants view drawings of humanfigures, each of which occurred with twostatements about the characteristics of theperson who drew it. Statements were paired withdrawings in such a way as to ensure no
relationship between drawings and personality
types, nevertheless participants saw the relationships they expected to see.
Nisbett and Ross (1980) summarized theChapmans' work on illusory correlation bysaying that "reported covariation was shown toreflect true covariation far less than it reflected
theories or preconceptions of the nature of theassociations that 'ought' to exist" (p. 97).
Goldberg (1968) said that this research "illustrates the ease with which one can 'learn'
relationships which do not exist" (p. 493). For
present purposes, it provides another illustrationof how the confirmation bias can work: thepresumption of a relationship predisposes one to183find evidence of that relationship, even when
there is none to be found or, if there is evidence
to be found, to overweight it and arrive at aconclusion that goes beyond what the evidence
justifies.
A form of stereotyping involves believing thatspecific behaviors are more common amongpeople who are members of particular groups
than among those who are not. There is aperceived correlation between group membership and behavior. Such perceived correlations
can be real or illusory. One possible explanationof the perception of illusory correlations is that
unusual behavior by people in distinctive groups
is more salient and easily remembered thansimilar behavior by people who are not members of those groups (Feldman, Camburn, &Gatti, 1986; Hamilton, Dugan, & Trolier, 1985).
Another possibility is that, once a person isconvinced that members of a specific group
behave in certain ways, he or she is more likely
to seek and find evidence to support the belief
than evidence to oppose it, somewhat independently of the facts.
Some investigators have argued that people
typically overestimate the degree to which
behavior in different situations can be predictedfrom trait variables or the degree to which anindividual's typical social behavior can bepredicted from a knowledge of that person's
behavior on a given occasion (Kunda & Nisbett,1986). An illusion of consistency, according tothis view, leads people to misjudge the extent towhich a friendly individual's behavior is consistently friendly or a hostile individual's behavior
is consistently hostile (Jennings, Amabile, &Ross, 1982; Mischel, 1968; Mischel & Peake,1982). It is easy to see how a pervasive
confirmation bias could be implicated in thisillusion: once one has categorized an individualas friendly or hostile, one will be more attuned
to evidence that supports that categorization
than to evidence that undermines it.
A more subtle problem relating to categorization involves the phenomenon of reification.
Taxonomies that are invented as conceptual
conveniences often come to be seen as representing the way the world is really structured. Given
the existence of a taxonomy, no matter how
arbitrary, there is a tendency to view the world interms of the categories it provides. One tends to184

RAYMOND S. NICKERSON

fit what one sees into the taxonomic bins athand. In accordance with the confirmation bias,
people are more likely to look for, and find,confirmation of the adequacy of a taxonomy
than to seek and discover evidence of its
limitations.

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Formal Reasoning and the Selection Task
I have already considered a task invented by
Wason and much used in rule-discovery experiments that revealed a tendency for people to testa hypothesized rule primarily by considering
instances that are consistent with it. Wason(1966, 1968) invented another task that also has
been widely used to study formal reasoning. In awell-known version of this task, participants see
an array of cards and are told that each card has aletter on one side and a number on the other.
Each of the cards they see shows either a vowel,
a consonant, an even number, or an odd number,and participants are asked to indicate whichcards one would have to turn over in order todetermine the truth or falsity of the following
statement: If a card has a vowel on one side thenit has an even number on the other side.
Suppose the array that people performing thistask see is as follows:

Given this set of cards, experimenters havegenerally considered selection of those showing
A and 7 to be correct, because finding an odd
number on the other side of the A or finding avowel behind the 7 would reveal the statement
to be false. The cards showing B and 4 havebeen considered incorrect selections, because
whatever is on their other sides is consistent
with the statement. In short, one can determine
the claim to be false by finding either the card
showing the A or the card showing the 7 to beinconsistent with it, or one can determine theclaim to be true by finding both of these cards tobe consistent with it. Wason found that people
performing this task are most likely to select
only the card showing a vowel or the card
showing a vowel and the one showing an evennumber; people seldom select either the card
showing a consonant or the one showing an odd
number. Numerous investigators have obtained
essentially the same result. Experiments withthis task and variants of it have beenreviewed many times (Cosmides, 1989; Evans,
1982; Evans, Newstead, & Byrne, 1993; Tweney & Doherty, 1983; Wason & Johnson-Laird,1972).
The logic of Wason's selection task is that of
the conditional: if P then Q. In the case of theabove example, P is there is a vowel on one side,and Q is there is an even number on the other.
Selecting the card showing the 7 is analogous tochecking to see if the not-Q case is accompanied
by not-P, as it must be if the conditional is true.
The basic finding of experiments with the task
lends support to the hypothesis—which is
supported also by other research (Evans, Newstead, & Byrne, 1993; Hollis, 1970)—that
people find the modus tollens argument (not-Q,
therefore not-P) to be less natural than themodus ponens form (P, therefore Q). And it isconsistent with the idea that, given the objectiveof assessing the credibility of a conditional
assertion, people are more likely to look for thepresence of the consequent given the presenceof the antecedent than for the absence of theantecedent given the absence of the consequent.
Several experiments have shown that performance of the selection task tends to beconsiderably better when the problem is couchedin familiar situational terms rather than abstractly (Johnson-Laird, Legrenzi, & Legrenzi,
1972; Wason & Shapiro, 1971), although it is by
no means always perfect in the former case
(Einhorn & Hogarth, 1978). People also generally do better when the task is couched in terms
that require deontic reasoning (deciding whether
a rule of behavior—e.g., permission, obligation,
promise—has been violated) rather than indicative reasoning (determining whether a hypothesis is true or false; Cheng & Holyoak, 1985;
Cosmides, 1989; Cosmides & Tooby, 1992;
Gigerenzer & Hug, 1992; Griggs & Cox, 1993;
Kroger, Cheng, & Holyoak, 1993; Manktelow &Over, 1990, 1991; Markovits & Savary, 1992;
Valentine, 1985; Yachanin & Tweney, 1982).
In relating confirmation bias to Wason's
selection task, it is important to make three
distinctions. The first is the distinction between
the objective of specifying which of four cards
in view must be turned over in order todetermine the truth or falsity of the assertion
with respect to those four cards and theobjective of saying which of the four types ofcards represented should be turned over in orderto determine the plausibility of the assertion

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CONFIRMATION BIAS

more generally. I believe that in most of theearlier experiments, the first of these tasks was
the intended one, although instructions have notalways made it explicit that this was the case.
The distinction is critical because what one
should do when given the first objective is clear
and not controversial. However the answer towhat one should do when given the second
objective is considerably more complicated anddebatable.
When the task is understood in the first way,
the only correct answer is the card showing thevowel and the one showing the odd number.
However, when one believes one is being asked
how one should go about obtaining evidence
regarding the truth or falsity of an assertion of
the form if P then Q where P and Q can beunderstood to be proxies for larger sets, it is nolonger so clear that the cards showing P and ~Q
are the best choices. Depending on the specificsof the properties represented by P and Q andwhat is known or assumed about their prevalence in the population of interest, P may be theonly selection that is likely to be very informative, and selection of ~Q may be essentiallypointless (Kirby, 1994b; Nickerson, 1996; Oaksford & Chater, 1994; Over & Evans, 1994).
Although experimenters have often, perhaps
more often than not, intended that the selection
task be interpreted in the first of the two ways
described, the second interpretation seems to me
more representative of the kind of conditional
reasoning that people are required to do ineveryday life; it is hard to think of everyday
examples of needing to determine the truth or
falsity of a claim about four entities like thecards in the selection task, all of which are
immediately available for inspection. Perhaps atendency to carry over to the task, uncritically,an effective approach in the world of everyday
hypothesis testing that is not always effective inthe contrived world of the laboratory contributes
to performance on the selection task.
The second and third distinctions pertain to
instances when the task is understood to involve
only the four cards in view. They are thedistinctions that were made in the context of the
discussion of how confirmation bias relates to"find-the-rule" tasks and other tasks involving
conditional syllogisms, namely the distinction
between evidence that is confirmatory by virtueof the presence of two entities rather than by
their joint absence and the distinction between

185

logical and psychological confirmation. As
applied to the selection task, confirmation bias
has typically connoted a tendency to look for thejoint occurrence of the items specified in thetask description. In the original version of thetask, this means looking for instances of thejoint occurrence of a vowel and an even number,and thus the turning over of the cards showing A
and 4, inasmuch as these are the only cards ofthe four shown that could have both of thenamed properties.
Selection of evidence that can only be
confirmatory of the hypothesis under consideration—or avoidance of evidence that could
possibly falsify the hypothesis—is one interpretation of the confirmation bias that is notconsistent with the selection of A and 4 inWason's task. A is a potentially falsifying card;
if the rule that cards that have a vowel on one
side have an even number on the other is false,
turning over the card showing A could reveal thefact. So in selecting A one is not avoiding thepossibility of falsification; one is performing anappropriate test, even in the strictest Popperiansense. The only way to avoid the possibility of
disconfirming the hypothesized rule in Wason's
task is to select either or both of the cards
showing B and 4, inasmuch as nothing that is on
the other sides of these cards could beinconsistent with it.
The fact that, when given the selection task,
people are at least as likely to select A as they are
to select 4 weighs against the idea that their
behavior can be attributed to a logically baseddesire to seek only confirming evidence and to
avoid potentially disconfirming evidence. I say
logically based because it may be that people
select A because they are seeking confirming
evidence, even though their selection also
provides the opportunity to acquire falsifying
evidence. From a strictly logical point of view,
although selecting A is part of the correct
solution, selecting only A, or A and 4, not only
fails to ensure the discovery that the hypothesized rule is false if it is false, it also fails toestablish its truth if it is true. The only way todemonstrate its truth is to rule out the possibility
that it is false, and this means checking all (both)cases (A and 7) that could possibly show it to befalse if it is.
So again, behavior that has been interpreted
as evidence of a confirmation bias is notstrongly confirmatory in a logical sense, al-This document is copyrighted by the American Psychological Association or one of its allied publishers.
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186

RAYMOND S. NICKERSON

though it may well be seen (erroneously) as soby those who display it. Inasmuch as anobservation can be confirming in a psychological sense (i.e., interpreted as confirming evidence) independently of whether it is logicallyconfirmatory, perhaps the confirmation bias
should be thought of as a tendency to seek
evidence that increases one's confidence in ahypothesis regardless of whether it should. The
distinction between logical confirmation andpsychological confirmation deserves greater
emphasis than it has received.
Although researchers have interpreted theconsiderable collection of results that have beenobtained from experiments with variations of
the selection task as generally reflective of aconfirmation bias, alternative hypotheses havealso been proposed. L. J. Cohen (1981) argued
that it is not necessary to suppose that people doanything more in testing a conditional rule thancheck whether the consequent holds true whenever the antecedent does. The fact that people
sometimes select the card representing Q (4 inthe case of the present example) as well as the
one representing P (A) is because they falsely
perceive ifP then Q to be equivalent to ifQ thenP (conversion error); and the fact that they do
not select the card representing not-Q (7 in thepresent example) is attributed to a failure to
apply the law of the contrapositive.
Several investigators have argued that peopleinterpret the conditional relationship ifP then Q
as equivalent to the biconditional relationship if
and only if P then Q, or that they fail todistinguish between ifP then Q and ifQ then P,
especially as the if-then construction is used ineveryday language (Chapman & Chapman,
1959; L. J. Cohen, 1981; Henle, 1962; Legrenzi,
1970; Politzer, 1986; Revlis, 1975a, 1975b).
Others have assumed the operation of amatching bias whereby people tend to focus on(and select) cards explicitly named in thestatement of the rule to be tested (Evans, 1972;
Evans & Lynch, 1973; Hoch & Tschirgi, 1983).
Evans (1989) has suggested that people maketheir selections, without thinking much about
them, on the basis of a "preconscious heuristic
judgment of relevance, probably linguisticallydetermined" (p. 108).
Recent theorizing about the selection task has
given rise to several new explanations of how
people interpret it and deal with it, especially aspresented in various concrete frames of refer-ence. An idea common to several of these
explanations is that people's performance of thetask is indicative of behavior that has proved tobe adaptively effective in analogous real-world
situations. Cosmides (1989), for example, hasargued that the typical results are consistent with
the idea that people are predisposed to look forcheating on a social contract and are particularly
skilled at detecting it. Consequently, they dowell on selection tasks in which checking thenot-Q condition is analogous to looking for an
instance of violating a tacit social agreement,
like failing to meet an obligation. Gigerenzer
and Hug (1992) have defended a similar point ofview.
Liberman and Klar (1996) proposed analternative explanation of the results of researchon cheating detection. They argued that acheating-detection perspective is neither necessary nor sufficient to yield the P and not-Q
selection; they contend that the logically correct
choice will be likely if the conditional ifP thenQ is understood to represent a deterministic (asopposed to a probabilistic) unidirectional relationship between P and Q, if what constitutes aviolation of the rule is clear, and if it isunderstood that the task is to look for such a
violation.
Other investigators have begun to view theselection task as more a problem of data
selection and decision making than of logicalinference (Kirby, 1994a, 1994b; Manktelow &Over, 1991, 1992). Oaksford and Chater (1994),
for example, have proposed a model of the task
according to which people should make selections so as to maximize their gain in information
regarding the tenability of the hypothesis thatthe conditional ifP then Q is true relative to thetenability that it is false. I have done an analysis
that leads to a similar conclusion, at least when
the task is perceived as that of deciding whether
the conditional is true in a general sense, as
opposed to being true of four specific cards(Nickerson, 1996).
Wason's selection task has proved to be oneof the most fertile paradigms in experimental
psychology. It would be surprising if all the
results obtained with the many variations of thetask could be accounted for by a single simple
hypothesis. I believe that, taken together, theresults of experimentation support the hypothesis that one of the several factors determining
performance on this task is a confirmation bias

CONFIRMATION BIAS

that operates quite pervasively. This is not tosuggest that a confirmation bias explains all thefindings with this task, or even that it is the mostimportant factor in all cases, but only that itexists and often plays a substantive role.

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The Primacy Effect and Belief Persistence
When a person must draw a conclusion on thebasis of information acquired and integrated
over time, the information acquired early in theprocess is likely to carry more weight than that
acquired later (Lingle & Ostrom, 1981; Sherman, Zehner, Johnson, & Hirt, 1983). This iscalled the primacy effect. People often form an
opinion early in the process and then evaluate
subsequently acquired information in a way thatis partial to that opinion (N. H. Anderson &Jacobson, 1965; Jones & Goethals, 1972;
Nisbett & Ross, 1980; Webster, 1964). Francis
Bacon (1620/1939) observed this tendency
centuries ago: "the first conclusion colors andbrings into conformity with itself all that comeafter" (p. 36).
Peterson and DuCharme (1967) had people
sample a sequence of colored chips and estimate
the probability that the sequence came from an
urn with a specified distribution of colors rather
than from a second urn with a different
distribution. The sampling was arranged so thatthe first 30 trials favored one urn, the second 30
favored the other, and after 60 trials the evidence
was equally strong for each possibility. Participants tended to favor the urn indicated by the
first 30 draws, which is to say that the evidence
in the first 30 draws was not countermanded by
the evidence in the second 30 even thoughstatistically it should have been.
Bruner and Potter (1964) showed people thesame picture on a series of slides. The first slide
was defocused so the picture was not recognizable. On successive slides the focus was madeincreasingly clear. After each presentation theparticipant was asked to identify what was
shown. A hypothesis formed on the basis of a
defocused picture persisted even after the
picture was in sufficiently good focus thatparticipants who had not looked at the poorly
focused picture were able to identify it correctly.
Jones, Rock, Shaver, Goethals, and Ward (1968)
had participants form opinions about people's
problem-solving abilities by watching them inaction. The problem solvers were judged to be

187

more competent if they solved many problems
early in a problem series and few late than ifthey did the reverse.
The primacy effect is closely related to (andcan perhaps be seen as a manifestation of) belief
persistence. Once a belief or opinion has been
formed, it can be very resistive to change, evenin the face of fairly compelling evidence that it
is wrong (Freedman, 1964; Hayden & Mischel,
1976; Luchins, 1942,1957; Rhine & Severance,
1970; Ross, 1977; Ross & Lepper, 1980; Ross,
Lepper, & Hubbard, 1975). Moreover it can bias
the evaluation and interpretation of evidence
that is subsequently acquired. People are more
likely to question information that conflicts withpreexisting beliefs than information that is
consistent with them and are more likely to see
ambiguous information to be confirming ofpreexisting beliefs than disconfirming of them(Ross & Anderson, 1982). And they can be quite
facile at explaining away events that areinconsistent with their established beliefs (Henrion & Fischhoff, 1986).
I have already discussed studies by Pitz
(1969) and Troutman and Shanteau (1977)
showing that people sometimes take normatively uninformative data as supportive of afavored hypothesis. One could interpret resultsfrom these studies as evidence of a beliefpreserving bias working in situations in which
the "information" in hand is not really informative. This is in keeping with the finding that two
people with initially conflicting views can
examine the same evidence and both findreasons for increasing the strength of their
existing opinions (Lord, Ross, & Lepper, 1979).
The demonstration by Pitz, Downing, andReinhold (1967) that people sometimes interpret
evidence that should count against a hypothesis
as counting in favor of it may be seen as anextreme example of the confirmation bias
serving the interest of belief preservation.
Ross and his colleagues have also shown
experimentally that people find it extremely
easy to form beliefs about or generate explanations of individuals' behavior and to perseverein those beliefs or explanations even after
learning that the data on which the beliefs or
explanations were originally based were fictitious (Ross et al., 1975; Ross, Lepper, Strack, &Steinmetz, 1977). Ross et al. (1975), forexample, had people attempt to distinguishbetween authentic and unauthentic suicide

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188

RAYMOND S. NICKERSON

notes. As the participants made their choices,
they were given feedback according to apredetermined schedule that was independent of
the choices they made, but that insured that the
feedback some participants received indicatedthat they performed far above average on thetask while that which others received indicatedthat they performed far below average.
Following completion of Ross et al.'s (1975)
task, researchers informed participants of thearbitrary nature of the feedback and told themthat their rate of "success" or "failure" waspredetermined and independent of their choices.
When the participants were later asked to rate
their ability to make such judgments, those who
had received much positive feedback on theexperimental task rated themselves higher than
did those who had received negative feedback,despite being told that they had been given
arbitrary information. A follow-up experiment
found similar perseverance for people who
observed others performing this task (but did not
perform it themselves) and also observed thedebriefing session.
Nisbett and Ross (1980) pointed out how aconfirmation bias could contribute to the perseverance of unfounded beliefs of the kind
involved in experiments like this. Receiving
feedback that supports the assumption that one
is particularly good or particularly poor at a task
may prompt one to search for additional
information to confirm that assumption. To theextent that such a search is successful, the belief
that persists may rest not exclusively on the
fraudulent feedback but also on other evidence
that one has been able to find (selectively) in
support of it.
It is natural to associate the confirmation bias
with the perseverance of false beliefs, but in fact
the operation of the bias may be independent of
the truth or falsity of the belief involved. Not
only can it contribute to the perseverance ofunfounded beliefs, but it can help make beliefs
for which there is legitimate evidence stronger
than the evidence warrants. Probably few beliefsof the type that matter to people are totally
unfounded in the sense that there is no
legitimate evidence that can be marshalled forthem. On the other hand, the data regarding
confirmation bias, in the aggregate, suggest thatmany beliefs may be held with a strength ordegree of certainty that exceeds what theevidence justifies.

Own-Judgment Evaluation
Many researchers have done experiments inwhich people have been asked to express theirdegree of confidence in judgments that they
have made. When participants have expressed
confidence as probability estimates or as ratings
that, with some plausible assumptions, can betransformed into probability estimates, it has
been possible to compare confidence withperformance on the primary task. Thus researchers can determine for each confidence judgment
the percentage of the correct items on the
primary task to which that judgment was
assigned. Plots of actual percent correct against
percent correct "predicted" by the confidence
judgments are often referred to as calibration
curves; perfect calibration is represented by theunit line, which indicates that for a given
confidence level, X, the proportion of all the
judgments with that level that were correct was X.
In general, people tend to express a higher
degree of confidence than is justified by theaccuracy of their performance on the primarytask, which is to say that calibration studies havetypically shown overconfidence to be morecommon than underconfidence (Einhorn &Hogarth, 1978; Fischhoff, 1982; Lichtenstein &Fischhoff, 1977; Lichtenstein, Fischhoff, &Phillips, 1977; Pitz, 1974; Slovic, Fischhoff, &Lichtenstein, 1977). Kahneman and Tversky(1973) refer to the confidence that people feelfor highly fallible performance as the illusion ofvalidity. Being forced to evaluate one's views,
especially when that includes providing reasons
against one's position, has reduced overconfidence in some instances (Fischhoff, 1977; Hoch,
1984, 1985; Koriat, Lichtenstein, & Fischhoff,
1980; Tetlock & Kim, 1987). But generally
overconfidence has only been reduced, noteliminated, and providing reasons against one's
position is not something that most people dospontaneously.
One explanation of overconfidence starts withthe assumption that people tend to be good
judges of their knowledge as it relates tosituations they are likely to encounter ineveryday life. This explanation of overconfidence also notes that a minimum requirementfor observing good calibration in experimental
situations is that the questions people are toanswer and that one to be used to judge theprobability of the correctness of their answers

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CONFIRMATION BIAS

189

must be selected in such a way as to ensure that clinical diagnoses based on case statistics tend
valid cues in the natural environment remain to be more accurate than those based on clinicalvalid in the experimental situation. Juslin (1993) judgments, clinicians typically have greater
has argued that certain strategies commonly confidence in their own judgments than in thoseused to select items in experiments more or less derived statistically from incidence data (Arkes,
ensure that valid knowledge from the partici- Dawes, & Christensen, 1986; Goldberg, 1968;
pants' natural environment will be less valid in Meehl, 1954, 1960; Sawyer, 1966). In contrast,
the experimental situation. More specifically, professional weather forecasters tend to be
the argument is that such selection strategies relatively well calibrated, at least with respect totypically result in sets of items for which cues their weather predictions (Murphy & Winkler,
leading to wrong answers are overrepresented 1974, 1977; Winkler & Murphy, 1968); this has
relative to their commonness in the natural been attributed to the fact that they receive
environment. Experiments showing good calibra- constant and immediate feedback regarding thetion for items selected at random from a set accuracy of their predictions, which is the kindassumed to be representative of the natural of information that makes learning feasible.
Griffin and Tversky (1992) have made aenvironment (Gigerenzer, Hoffrage, & Kleinbolting, 1991; Juslin, 1993, 1994) support this distinction between strength (extremeness) ofevidence and weight (predictive validity) of
explanation.
evidence. They hypothesized that people tend toI believe this argument to be a forceful one. focus primarily on strength and make some
The evidence is fairly strong that some of the (typically insufficient) adjustments in response
overconfidence reported, especially in experi- to weight. This hypothesis leads to the expectaments that require people to answer general- tion that overconfidence will be the ruleknowledge questions in a forced-choice format, especially when strength is high and weight low,
is an artifact of item-selection procedures that whereas underconfidence becomes more likely
do not ensure that cues have the same validity in when the opposite is the case. This account isexperimental situations that they have in typical consistent with the finding that overconfidence
real-life situations. Not all studies of confidence is the general rule because of the assumed
or calibration have used general-knowledge greater importance attached to evidentiary
questions and the forced-choice format for strength. The account is even more predictive ofanswers, however, and overconfidence has also general overconfidence if one assumes thatbeen observed in other contexts. Researchers strong evidence is more common than weighty
have shown that assessments of peoples' ability evidence, as Griffin and Tversky use these terms.
to recognize faces, for example, correlate poorly
Another account of why people tend to bewith performance on a face-recognition task overconfident of their own knowledge is that
(Baddeley & Woodhead, 1983) and observed when one has produced a tentative answer to athat retrospective judgments of comprehension question, one's natural inclination is to searchof expository texts are higher than justified memory for evidence to support that answer and(Glenberg, Wilkinson, & Epstein, 1982).
to fail to consider possible alternative answers
Experts are not immune from the illusion of (Graesser & Hemphill, 1991; Griffin, Dunning,validity. Attorneys tend, in the aggregate, to & Ross, 1990; Hoch, 1985; Koriat et al., 1980;
express confidence of obtaining outcomes better Shaklee & Fischhoff, 1982). This is similar tothan those they actually obtain (Loftus & the explanation already mentioned of why
Wagenaar, 1988; Wagenaar & Keren, 1986). people sometimes persevere with a belief even
Other professionals who have been found to be after learning that the information on which theoverconfident when making judgments in their belief was based was fictitious: after having
own areas of expertise include physicians formed the belief they sought and found(Lusted, 1977), psychologists (Oskamp, 1965), independent data to corroborate it.
and engineers (Kidd, 1970). Experts appear to
do better when there is a reliable basis for
The Confirmation Biasstatistical prediction, such as when predicting
in Real-World Contexts
bridge hands (Keren, 1987) or betting odds forhorse racing (Hausch, Ziemba, & Rubenstein,
As the foregoing review shows, the confirma1981). On the other hand, despite the fact that tion bias has been observed in a variety of guises

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in many experimental situations. What makes it
especially important to understand is that it canhave significant consequences in many nonlaboratory contexts. The point is illustrated in what
follows by a few examples.

height was roughly the same as the ratio of thediameter of a circle to its circumference, whichis to say, IT. Smyth was inspired by Taylor's
observations and set himself the task of
discovering other mathematical relationships ofinterest that the monument might hide.
Smyth discovered that the ratio of the
Number Mysticism
pyramid's base to the width of a casing stonethat
Pythagoras discovered, by experimentation, was 365, the number of days in a year, and9
how the pitch of a sound emitted by a vibrating the pyramid's height multiplied by 10 was
string depends on the length of the string and approximately equal to the distance from thewas able to state this dependency in terms of earth to the sun. By comparing pyramid length
simple numerical ratios: (a) two strings of the measurements in various ways, he was able tosame material under the same tension differ in find numbers that correspond to many quantitapitch by an octave when one of the strings is tive properties of the world that were presumtwice as long as the other, and (b) two strings the ably unknown when the pyramid was built.
lengths of which are in the ratio 2 to 3 will These include the earth's mean density, theperiod of precession of the earth's axis, and the
produce a note and its fifth, and so on.
Observation was not the new thing that mean temperature of the earth's surface. Von
Pythagoras did in his study of harmonic Daniken (1969) used the existence of such
relationships; people had been observing the relationships as the basis for arguing that theheavens and recording what they saw for a long earth had been visited by intelligent extraterrestime. What he did that was new was manipulate trials in the past.
Gardner (1957) referred to Smyth's book as awhat he was observing and take notice of theeffects of those manipulations. He has been classic of its kind illustrating beautifully "thecalled the first experimentalist. Ironically, in- ease with which an intelligent man, passionately
stead of establishing experimentation as an convinced of a theory, can manipulate his
especially fruitful way to investigate the proper- subject matter in such a way as to make itties of the physical world, Pythagoras's discov- conform to precisely held opinions" (p. 176). He
ery helped to usher in what Bell called "the pointed out that a complicated structure like thegolden age of number mysticism" and to delay pyramid provides one with a great assortment of
the acceptance of experimentation as the pri- possible length measurements, and that anyone
mary method of science for 2000 years (Bell, with the patience to juggle them is quite sure to
1946/1991). Pythagoras's intellectual heirs were find a variety of numbers that will coincide withso convinced of the validity of his pronounce- some dates or figures that are of interest for
ment, "everything is number," that many of the historical or scientific reasons. One simply
most able thinkers over the next 2 millenia makes an enormous number of observations,devoted much of their cognitive energy to the tries every manipulation on measurements andpursuit of numerology and the confirmation of measurement relationships that one can imagits basic assumptions. It took a Galileo to kindle ine, and then selects from the results those few
an interest in experimentation that would not that coincide with numbers of interest in othercontexts. He wrote, "Since you are bound by noagain sputter and die.
Work done as recently as the mid-19th rules, it would be odd indeed if this search for
century involving the great pyramid of Egypt, Pyramid 'Truths' failed to meet with considerwhich has extraordinary fascination for modern able success" (p. 177). The search for pyramid
observers of artifacts of ancient cultures, illus- truths is a striking illustration of how a bias totrates the relevance of the confirmation bias to confirm is expressed by selectivity in the search
Pythagorean numerological pursuits. Much of for and interpretation of information.
this fascination is due to certain mathematical
relationships discussed first by John Taylor Witch Hunting(1859) and shortly later by Charles Smyth(1864/1890). Taylor noted, among other facts,
From a modern vantage point, the convictions
that the ratio of twice the pyramid's base to its and executions of tens of thousands of individu-

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CONFIRMATION BIAS

als for practicing witchcraft during the 15th,
16th, and 17th centuries in Western Europe andto a lesser extent in 18th-century New England,
is a particularly horrific case of the confirmation
bias functioning in an extreme way at thesocietal level. From the perspective of many,
perhaps most, of the people of the time, belief inwitchcraft was perfectly natural and sorcery was
widely viewed as the reason for ills and troubles
that could not otherwise be explained. Executioners meted out punishment for practising witchcraft—generally the stake or the scaffold—withappalling regularity. Mackay (1852/1932) put
the number executed in England alone, during
the first 80 years of the 17th century, at 40,000.
People believed so strongly in witchcraft thatsome courts had special rules of evidence toapply only in cases involving it. Mackay(1852/1932) quoted Bodinus, a 17th-century
French authority, as follows:
The trial of this offense must not be conducted likeother crimes. Whoever adheres to the ordinary courseof justice perverts the spirit of the law, both divine andhuman. He who is accused of sorcery should never beacquitted, unless the malice of the prosecutor be clearer
than the sun; for it is so difficult to bring full proof ofthis secret crime, that out of a million witches not one
would be convicted if the usual course were followed!
(p. 528)

Torture was an accepted and widely practicedmeans of exacting confessions from accused
persons. Failure to denounce a witch was insome places a punishable offense.
It is hard for those who live in a society thatrecognizes the principle that a person accused ofa crime is to be presumed innocent until provenguilty beyond a reasonable doubt to imagine
what it must have been like to live at a time
when—at least with respect to the accusation ofwitchcraft—just the opposite principle held. On
the other hand, it is perhaps too easy to assume
that nothing like the witchcraft mania could
occur in our enlightened age. A moment's
reflection on the instances of genocide or
attempted genocide that have occurred in recent
times should make people wary of any suchassumption. These too can be seen as instancesof what can happen when special rules ofevidence are used to protect and support beliefs
that individuals, groups or nations want, forwhatever reasons, very much to hold. Awarenessof a prevalent bias toward confirmation evenunder relatively ideal conditions of evidence

191

evaluation should make people doubly cautious
about the potential for disaster when circumstances encourage the bias to be carried toexcess.

Policy Rationalization
Tuchman (1984) described a form of confirmation bias at work in the process of justifying
policies to which a government has committed
itself: "Once a policy has been adopted andimplemented, all subsequent activity becomes
an effort to justify it" (p. 245). In the context of a
discussion of the policy that drew the UnitedStates into war in Vietnam and kept the U.S.
military engaged for 16 years despite countlessevidences that it was a lost cause from the
beginning, Tuchman argued that once a policy
has been adopted and implemented by agovernment, all subsequent activity of thatgovernment becomes focused on justification ofthat policy:
Wooden headedness, the source of self deception is afactor that plays a remarkably large role in government.
It consists in assessing a situation in terms of
preconceived fixed notions while ignoring or rejecting
any contrary signs. It is acting according to wish while
not allowing oneself to be deflected by the facts. It is
epitomized in a historian's statement about Philip II of
Spain, the surpassing wooden head of all sovereigns:"no experience of the failure of his policy could shakehis belief in its essential excellence." (p. 7)

Folly, she argued, is a form of self-deception
characterized by "insistence on a rooted notion
regardless of contrary evidence" (p. 209).
At the beginning of this article, I gave
examples of bias in the use of information thatwould not be considered illustrative of theconfirmation bias, as that term is used here.
These examples involved intentional selectivity
in the use of information for the conscious
purpose of supporting a position. Anotherexample is that of politicians taking note of facts
that are consistent with positions they havetaken while intentionally ignoring those that areinconsistent with them. This is not to suggest,
however, that all selective use of information in
the political arena is knowing and deliberate andthat confirmation bias, in the sense of unwitting
selectivity, is not seen here. To the contrary, I
suspect that this type of bias is especiallyprevalent in situations that are inherently
complex and ambiguous, which many political

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situations are. In situations characterized byinteractions among numerous variables and inwhich the cause-effect relationships are obscure, data tend to be open to many interpretations. When that is the case, the confirmation
bias can have a great effect, and people should
not be surprised to see knowledgeable, wellintentioned people draw support for diametrically opposed views from the same set of data.

Medicine
The importance of testing theories, hypotheses, speculations, or conjectures about theworld and the way it works, by understanding
their implications vis-a-vis observable phenomena and then making the observations necessary
to check out those implications, was a common
theme in the writings of the individuals who
gave science its direction in the 16th, 17th, and18th centuries. This spirit of empirical criticism
had not dominated thinking in the preceding
centuries but was a new attitude. Theretofore
observation, and especially controlled experimentation, had played second fiddle to logic andtradition.
Consider, for example, the stagnated status of
medical knowledge: "For 1500 years the main
source of European physicians' knowledge
about the human body was not the body itself. . . [but] the works of an ancient Greek
physician [Galen]. 'Knowledge' was the barrier
to knowledge. The classic source became arevered obstacle." Boorstin (1985, p. 344), whowrote these words, noted the irony of the factthat Galen himself was an experimentalist andurged others who wished to understand anatomy
or medicine to become hands-on investigatorsand not to rely only on reading what others had
said. But Galen's readers found it easier to relyon the knowledge he passed on to them than tofollow his advice regarding how to extend it.
Although some physicians did "experiment"with various approaches to the treatment ofdifferent diseases and ailments, as Thomas(1979) pointed out, until quite recently, thisexperimentation left something to be desired asa scientific enterprise:
Virtually anything that could be thought up for thetreatment of disease was tried out at one time oranother, and, once tried, lasted decades or evencenturies before being given up. It was, in retrospect,
the most frivolous and irresponsible kind of humanexperimentation, based on nothing but trial and error,
and usually resulting in precisely that sequence.
Bleeding, purging, cupping, the administration of
infusions of every known plant, solutions of every
known metal, every conceivable diet including total
fasting, most of these based on the weirdest imaginings
about the cause of disease, concocted out of nothing but
thin air—this was the heritage of medicine up until alittle over a century ago. It is astounding that the
profession survived so long, and got away with somuch with so little outcry. Almost everyone seems tohave been taken in. (p. 159)

How is it that ineffective measures could becontinued for decades or centuries without theirineffectiveness being discovered? Sometimes
people got better when they were treated;sometimes they did not. And sometimes they got
better when they were not treated at all. It
appears that people's beliefs about the efficacyof specific treatments were influenced morestrongly by those instances in which treatment
was followed by recovery than in either those inwhich it was not or those in which it occurred
spontaneously. A prevailing tendency to focus
exclusively or primarily on positive cases—
cases in which treatment was followed byrecovery—would go a long way toward accounting for the fact that the discoveries that certain
diseases have a natural history and people often
recover from them with or without treatment
was not made until the 19th century.
Fortunately such a tendency is no longer
characteristic of medical science as a whole; but
one would be hard pressed to argue that it nolonger influences the beliefs of many people
about the efficacy of various treatments ofmedical problems. Every practitioner of a formof pseudomedicine can point to a cadre ofpatients who will testify, in all sincerity, tohaving benefited from the treatment. Moregenerally, people engage in specific behaviors(take a pill, rest, exercise, think positively) forthe express purpose of bringing about a specifichealth-related result. If the desired result occurs,
the natural tendency seems to be to attribute it towhat was done for the purpose of causing it;
considering seriously the possibility that theresult might have been obtained in the absenceof the associated "cause" appears not to comenaturally to us but to have to be learned.
Medical diagnosis, as practiced today, hasbeen the subject of some research. In looking forcauses of illness, diagnosticians tend to generate
one or a small set of hypotheses very early in theThis document is copyrighted by the American Psychological Association or one of its allied publishers.
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CONFIRMATION BIAS

diagnostic session (Elstein, Shulman, & Sprafka,
1978). The guidance that a hypothesis in hand
represents for further information gathering canfunction as a constraint, decreasing the likelihood that one will consider an alternative
hypothesis if the one in hand is not correct.
Failure to generate a correct hypothesis has been
a common cause of faulty diagnosis in somestudies (Barrows, Feightner, Neufeld, & Norman, 1978; Barrows, Norman, Neufeld, &Feightner, 1977). A hypothesis in hand also canbias the interpretation of subsequently acquired
data, either because one selectively looks fordata that are supportive of the hypothesis andneglects to look for disconfirmatory data (Barrows et al., 1977) or because one interprets data
to be confirmatory that really are not (Elstein etal., 1978).
Studies of physicians' estimates of the probability of specific diagnoses have often shown
estimates to be too high (Christensen-Szalanski& Bushyhead, 1981/1988; DeSmet, Fryback, &Thornbury, 1979). Christensen-Szalanski andBushyhead (1981/1988) had physicians judge,
on the basis of a medical history and physicalexamination, the probability that patients at awalk-in clinic had pneumonia. Only about 20
percent of the patients who were judged to havepneumonia with a probability between .8 and .9
actually had pneumonia, as determined by chest
X rays evaluated by radiologists. These investigators also got physicians to rate the various
possible outcomes from such a diagnosis andfound no difference between the values assigned
to the two possible correct diagnoses and nodifference between the values assigned to thepossible incorrect ones. This suggests that theoverly high estimates of probability of disease
were not simply the result of a strong preferenceof false positive over false negative results.
Results from other studies suggest thatphysicians do not do very well at revisingexisting probability estimates to take into
account the results of diagnostic tests (Berwick,Fineberg, & Weinstein, 1981; Cassells, Schoenberger, & Grayboys, 1978). A diagnostic findingthat fails to confirm a favored hypothesis may bediscounted on the grounds that, inasmuch as
there is only a probabilistic relationship between
symptoms and diseases, a perfect match is not to
be expected (Elstein & Bordage, 1979).

193

Judicial Reasoning
In the context of judicial proceedings, anattempt is made to decouple the process ofacquiring information from that of drawing
conclusions from that information. Jurors areadmonished to maintain open minds during thepart of a trial when evidence is being presented(before the jury-deliberation phase); they are notsupposed to form opinions regarding what theverdict should be until all the evidence has beenpresented and they have been instructed by thejudge as to their decision task. During thejury-deliberation phase, the jury's task is toreview and consider, collectively, the evidence
that has been presented and to arrive, throughdiscussion and debate, at a consensus on theverdict. They are to be careful to omit fromconsideration any evidence that came to lightduring the trial that was deemed inadmissible
and ordered stricken from the record.
The admonition to maintain an open mind
during the evidence-presentation phase of a trial
seems designed to counter the tendency to form
an opinion early in an evidence-evaluation
process and then to evaluate further evidencewith a bias that favors that initial opinion.
Whether jurors are able to follow this admonition and to delay forming an opinion as to thetruth or falsity of the allegations against theaccused until the jury-deliberation phase is amatter of some doubt. It is at least a plausible
possibility that individual jurors (and judges)
develop their personal mental models of "whatreally happened" as a case develops andcontinuously refine and elaborate those models
as evidence continues to be presented (Holstein,1985). To the extent that this is the way jurors
actually think, a model, as it exists at any point
in time, may strongly influence how new
information is interpreted. If one has come tobelieve that a defendant is guilty (or innocent),
further evidence that is open to various interpretations may be seen as supportive of that belief.
Opinions formed about a defendant on the basisof superficial cues (such as demeanor while
giving testimony) can bias the interpretation of
inherently ambiguous evidence (Hendry &Shaffer, 1989).
Results of mock-trial experiments indicate
that, although there are considerable individual
differences among mock jurors with respect toThis document is copyrighted by the American Psychological Association or one of its allied publishers.
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194

RAYMOND S. NICKERSON

how they approach their task (D. Kuhn et al.,1994), jurors often come to favor a particularverdict early in the trial process (Devine &Ostrom, 1985). The final verdicts that juries
return are usually the same as the tentative ones
they initially form (Kalven & Zeisel, 1966;
Lawson, 1968). The results of some mock trials
suggest that deliberation following the presentation of evidence tends to have the effect ofmaking a jury's average predeliberation opinion
more extreme in the same direction (Myers &Lamm, 1976).
The tendency to stick with initial tentative
verdicts could exist because in most cases theinitial conclusions stand up to further objective
evaluation; there is also the possibility, however,that the tentative verdict influences jurors'
subsequent thinking and biases them to look for,or give undo weight to, evidence that supports it.
This possibility gains credence from the finding
by Pennington and Hastie (1993) that participants in mock-jury trials were more likely toremember statements consistent with theirchosen verdict as having been presented as trial
evidence than statements that were inconsistent
with this verdict. This was true both ofstatements that had been presented (hits) and of
those that had not (false positives).

SciencePolya (1954a) has argued that a tendency toresist the confirmation bias is one of the ways inwhich scientific thinking differs from everyday
thinking:
The mental procedures of the trained naturalist are notessentially different from those of the common man,but they are more thorough. Both the common man andthe scientist are led to conjectures by a few observations and they are both paying attention to later cases
which could be in agreement or not with the conjecture.
A case in agreement makes the conjecture more likely,
a conflicting case disproves it, and here the difference
begins: Ordinary people are usually more apt to look
for the first kind of cases, but the scientist looks for thesecond kind. (p. 40)

If seeking data that would disconfirm ahypothesis that one holds is the general ruleamong scientists, the history of science gives us
many exceptions to the rule (Mahoney, 1976,
1977; Mitroff, 1974). Michael Faraday was
likely to seek confirming evidence for ahypothesis and ignore such disconfirming evidence as he obtained until the phenomenon

under study was reasonably well understood, atwhich time he would begin to pay more
attention to disconfirming evidence and actively
seek to account for it (Tweney & Doherty,1983). Louis Pasteur refused to accept or
publish results of his experiments that seemed totell against his position that life did not generatespontaneously, being sufficiently convinced ofhis hypothesis to consider any experiment that
produced counterindicative evidence to benecessarily flawed (Farley & Geison, 1974).
When Robert Millikan published the experimental work on determining the electric charge of asingle electron, for which he won the Nobel
prize in physics, he reported only slightly morethan half (58) of his (107) observations, omittingfrom publication those that did not fit his
hypothesis (Henrion & Fischhoff, 1986).
It is not so much the critical attitude that
individual scientists have taken with respect totheir own ideas that has given science thesuccess it has enjoyed as a method for making
new discoveries, but more the fact that individual scientists have been highly motivated todemonstrate that hypotheses that are held bysome other scientist(s) are false. The insistenceof science, as an institution, on the objective
testability of hypotheses by publicly scrutable
methods has ensured its relative independence
from the biases of its practitioners.
Conservatism among scientists. The factthat scientific discoveries have often met
resistance from economic, technological, religious, and ideological elements outside sciencehas been highly publicized. That such discoveries have sometimes met even greater resistancefrom scientists, and especially from those whose
theoretical positions were challenged or invalidated by those discoveries, is no less a fact ifless well known (Barber, 1961; Mahoney, 1976,1977). Galileo, for example, would not accept
Kepler's hypothesis that the moon is responsible
for the tidal motions of the earth's oceans.
Newton refused to believe that the earth could
be much older than 6,000 years on the strengthof the reasoning that led Archbishop Usher toplace the date of creation at 4,004 BC. Huygens
and Leibniz rejected Newton's concept ofuniversal gravity because they could not accept
the idea of a force extending throughout space
not reducible to matter and motion.
Humphrey Davy dismissed John Dalton's
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CONFIRMATION BIAS

more ingenious than important. William Thomson (Lord) Kelvin, who died in 1907, someyears after the revolutionary work of Joseph
Thomson on the electron, never accepted theidea that the atom was decomposable into
simpler components. Lev Landau was willing in1932 to dismiss the laws of quantum mechanics
on the grounds that they led to such a ridiculous
prediction as the contraction of large burned out
stars to essentially a point. Arthur Eddington
rejected Subrahmanyan Chandrasekhar's prediction in the early 1930s that cold stars with amass of more than about one half that of the sun
would collapse to a point. Chandrasekhar was
sufficiently discouraged by Eddington's reaction
to leave off for the better part of his professional
career the line of thinking that eventually led to
the now widely accepted theory of black holes.
Theory persistence. The history of science
contains many examples of individual scientists
tenaciously holding on to favored theories longafter the evidence against them had become
sufficiently strong to persuade others without
the same vested interests to discard them. I. B.
Cohen (1985) has documented many of thesestruggles in his account of the role of revolutionin science. All of them can be seen as examplesof the confirmation bias manifesting itself as anunwillingness to give the deserved weight toevidence that tells against a favored view.
Roszak (1986) described the tenacity withwhich adherents to the cosmology of Ptolemyclung to their view in the face of mounting
evidence of its untenability in this way:
The Ptolemaic cosmology that prevailed in ancient
times and during the Middle Ages had been compromised by countless contradictory observations overmany generations. Still, it was an internally coherent,
intellectually pleasing idea; therefore, keen minds
stood by the familiar old system. Where there seemed
to be any conflict, they simply adjusted and elaborated
the idea, or restructured the observations in order tomake them fit. If observations could not be made to fit,
they might be allowed to stand along the culturalsidelines as curiosities, exceptions, freaks of nature. It
was not until a highly imaginative constellation of ideas
about celestial and terrestrial dynamics, replete withnew concepts of gravitation, inertia, momentum, andmatter, was created that the old system was retired,(p. 91)

Roszak pointed out also that scientists throughout the 18th and 19th centuries retained otherinherited ideas in the fields of chemistry,

195

geology, and biology in adjusted forms, despite
increasing evidences of their inadequacies.
Science has held, often for very long times,
some beliefs and theories that could have been
invalidated if a serious effort had been made toshow them to be false. The belief that heavier
bodies fall faster than lighter ones, for example,
prevailed from the time of Aristotle until that ofGalileo. The experiments that Galileo performed to demonstrate that this belief was false
could have been done at any time during that
2000-year period. This is a particularly interesting example of a persisting false belief because
it might have been questioned strictly on thebasis of reasoning, apart from any observations.
Galileo posed a question that could have beenasked by Aristotle or by anybody who believedthat heavier bodies fall faster than lighter ones:
If a 10 pound weight falls faster than a 1 pound
weight, what will happen when the two are tied
together? Will the 11 pound combination fall
faster than the 10 pound weight, or will it fall
more slowly because the 1 pound weight willhold back the 10 pound one?
Hawking (1988) argued that the fact that theuniverse is expanding could have been predictedfrom Newton's theory of gravity at any timeafter the late 17th century, and noted that
Newton himself realized that the idea of
universal gravity begged an explanation as towhy the stars did not draw together. The
assumption that the universe was static was astrong and persistent one, however. Newton
dealt with the problem by assuming the universe
was infinite and consequently had no center onto
which it could collapse. Others introduced anotion that at very great distances gravity
became repulsive instead of attractive. Einstein,in order to make his general theory of relativity
compatible with the idea of a static universe,
incorporated a "cosmological constant" in thetheory which, in effect, nullified the otherwise
expected contraction. Einstein later was embarrassed by this invention and saw it as his greatest
mistake.
None of this is to suggest that scientists
accept evidences of the inadequacy of anestablished theory with equanimity. Rather, I
note that the typical reaction to the receipt ofsuch evidences is not immediately to discard thetheory to which the inadequacies relate but tofind a way to defend it. The bias is definitely inthe direction of giving the existing theory theThis document is copyrighted by the American Psychological Association or one of its allied publishers.
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196

RAYMOND S. NICKERSON

benefit of the doubt, so long as there is room for
doubt and, in some cases, even when there isnot. The usual strategy for dealing withanomalous data is first to challenge the data
themselves. If they prove to be reliable, the next
step is to complicate the existing theory justenough to accommodate the anomalous result
to, as T. S. Kuhn (1970) put it, "devise
numerous articulations and ad hoc modifications of [the] theory in order to eliminate any
apparent conflict" (p. 78). If that proves to be
too difficult, one may decide simply to live withthe anomaly, at least for a while. When a theory
is confronted with too many anomalies to beaccommodated in this way—or when, as aconsequence of a series of modifications thetheory becomes too convoluted to manage andan alternative theory becomes available—thereis the basis of a paradigm shift and arevolutionary reorientation of thinking.
Overconfidence. Overconfidence in experimental results has manifested itself in thereporting of a higher-than-warranted degree ofcertainty or precision in variable measurements.
Scientific investigators often have underestimated the uncertainty of their measurements andthus reported errors of estimate that have notstood the test of time. Fundamental constants
that have been reported with uncertainty estimates that later proved too small include thevelocity of light, the gravitational constant, andthe magnetic moment of the proton (Henrion &Fischhoff, 1986).
The 1919 British expedition to West Africa totake advantage of a solar eclipse in order to test
Einstein's prediction that the path of light would
be bent by a gravitational field represents anespecially noteworthy case of the reporting of ahigher-than-warranted degree of precision inmeasurement. Einstein had made the predictionin the 1915 paper on the general theory of
relativity. Scientists later discovered that theerror of measurement was as great as the effectthat was being measured so that, as Hawking(1988) put it, "The British team's measurement
had been sheer luck, or a case of knowing theresult they wanted to get, not an uncommon
occurrence in science" (p. 32).
The predictions have subsequently beenverified with observations not subject to thesame measurement problems, but as first madeand reported, they suggest the operation a
confirmation bias of considerable strength. In adetailed account of the event, Collins and Pinch(1993) noted that Eddington's data were noisy,
that he had to decide which photographs tocount and which to ignore, and that he used
Einstein's theory to make these decisions. As
they put it:
Eddington could only claim to have confirmed Einsteinbecause he used Einstein's derivations in deciding what
his observations really were, while Einstein's derivations only became accepted because Eddington'sobservation seemed to confirm them. Observation andprediction were linked in a circle of mutual confirmation rather than being independent of each other as wewould expect according to the conventional idea of anexperimental test. (p. 45)

Collins and Pinch's account of the reportingof the results of the 1919 expedition and of thesubsequent widespread adoption of relativity as
the new standard paradigm of physics representsscientific advance as somewhat less inexorably
determined by the cold objective assessment oftheory in the light of observational fact than it issometimes assumed to be.
Henrion and Fischhoff (1986) suggested thatthe overconfidence associated with the estimates
they considered could have resulted fromscientists overlooking, for one reason or another, specific sources of uncertainty in theirmeasurements. This possibility is consistent
with the results of laboratory studies of judgment showing that people typically find it easierto think of reasons that support a conclusion
they have drawn than to think of reasons that
contradict it and that people generally havedifficulty in thinking of reasons why their best
guess might be wrong (Koriat et al., 1980).
By way of rounding out this discussion ofconfirmation bias in science, it is worth notingthat prevailing attitudes and opinions canchange rapidly within scientific communities, as
they can in other communities. Today's revolutionary idea is tomorrow's orthodoxy. Ideas
considered daring, if not bizarre or downrightridiculous when first put forward, can become
accepted doctrine or sometimes obvious truths
that no reasonable person would contest in
relatively short periods of time. According toLakatos (1976)
Newton's mechanics and theory of gravitation was put
forward as a daring guess which was ridiculed andcalled "occult" by Leibniz and suspected even by
Newton himself. But a few decades later—in absenceof refutations—his axioms came to be taken as
indubitably true. Suspicions were forgotten, critics

CONFIRMATION BIAS
branded "eccentric" if not "obscurantist"; some of his
most doubtful assumptions came to be regarded as so
trivial that textbooks never even stated them. (p. 49,
Footnote 1)

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One can see a confirmation bias both in the
difficulty with which new ideas break throughopposing established points of view and in the
uncritical allegiance they are often given once
they have become part of the established view
themselves.

Explanations of the Confirmation Bias
How is one to account for the confirmation
bias and its prevalence in so many guises? Is it amatter of protecting one's ego, a simple
reluctance to consider the possibility that abelief one holds or a hypothesis that one isentertaining is wrong? Is it a consequence ofspecific cognitive limitations? Does it reflect alack of understanding of logic? Does it persist
because it has some functional value? That is,does it provide some benefits that are asimportant as, or in some situations more
important than, an attempt to determine the truth
in an unbiased way would be?

The Desire to Believe
Philosophers and psychologists alike haveobserved that people find it easier to believe
propositions they would like to be true thanpropositions they would prefer to be false. Thistendency has been seen as one manifestation ofwhat has been dubbed the Pollyanna principle(Matlin & Stang, 1978), according to whichpeople are likely to give preferential treatment
to pleasant thoughts and memories over unpleasant ones.
Finding a positive correlation between theprobability that one will believe a proposition to
be true and the probability that one will consider
it to be desirable (Lefford, 1946; McGuire,
1960; Weinstein, 1980, 1989) does not, in itself,
establish a causal link between desirability andperceived truth. The correlation could reflect a
relationship between truth and desirability in thereal world, whereby what is likely to be true islikely also to be desirable, and the same isconversely true. On the other hand, the evidence
is strong that the correlation is the result, at least
to some degree, of beliefs being influenced by
preferences. The continuing susceptibility of

197

people to too-good-to-be-true promises of quick
wealth is but one illustration of the fact thatpeople sometimes demand very little in the wayof compelling evidence to drive them to aconclusion that they would like to accept.
Although beliefs can be influenced by preferences, there is a limit to how much influencepeople's preferences can have. It is not the case,for most of us at least, that we are free to believe
anything we want; what we believe must appear
to us believable. We can be selective withrespect to the evidence we seek, and we can tilt
the scales when we weigh what we find, but wecannot completely ignore counterindicative evidence of which we are aware. Kunda (1990) has
made this argument persuasively. The very factthat we sometimes seek to ignore or discountevidence that counts against what we would liketo believe bears witness to the importance weattach to holding beliefs that are justified.
More generally, one could view, somewhat
ironically perhaps, the tendency to treat data
selectively and partially as a testament to thehigh value people attach to consistency. If
consistency between beliefs and evidence wereof no importance, people would have no reason
to guard beliefs against data that are inconsistent
with them. Consistency is usually taken to be an
important requirement of rationality, possibly
the most important such requirement. Paradoxically, it seems that the desire to be consistent canbe so strong as to make it difficult for one toevaluate new evidence pertaining to a statedposition in an objective way.
The quote from Mackay (1852/1932) that isused as an epigraph at the beginning of thisarticle stresses the importance of motivation inefforts to confirm favored views. Some investigators have argued, however, that the basic
problem is not motivational but reflects limitations of a cognitive nature. For example, Nisbett
and Ross (1980) held that, on the whole,investigators have been too quick to attribute thebehavior of participants in experimental situations to motivational biases when there wereequally plausible alternative interpretations of
the findings:
One wonders how strongly the theory of self-serving
bias must have been held to prompt such uncriticalacceptance of empirical evidence. . . . We doubt that
careful investigation will reveal ego-enhancing or
ego-defensive biases in attribution to be as pervasive or

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potent as many lay people and most motivational
theorists presume them to be. (p. 233)

tions as causal factors. It is possible, however,and probable, in my view, that both motivationaland cognitive factors are involved and that eachtype can mediate effects of the other.

This argument is especially interesting in thepresent context, because it invokes a form ofconfirmation bias to account for the tendency ofsome investigators to attribute certain behaviors Information-Processing Bases
to motivational causes and to ignore what, in for Confirmation Bias
Nisbett and Ross's view, are equally likely
The confirmation bias is sometimes attributed
alternative explanations.
The role of motivation in reasoning has been in part to the tendency of people to gather
a subject of debate for some time. Kunda (1990) information about only one hypothesis at a time
noted that many of the phenomena that once and even with respect to that hypothesis towere attributed to motivational variables have consider only the possibility that the hypothesis
been reinterpreted more recently in cognitive is true (or only the possibility that it is false) but
terms; according to this interpretation, conclu- not to consider both possibilities simultaneously
sions that appear to be drawn only because (Tweney, 1984; Tweney & Doherty, 1983).
people want to draw them may be drawn Doherty and Mynatt (1986) argued, for exbecause they are more consistent with prior ample, that people are fundamentally limited tobeliefs and expectancies. She noted too that think of only one thing at a time, and oncesome theorists have come to believe that having focused on a particular hypothesis, they
motivational effects are mediated by cognitive continue to do so. This, they suggested, explains
processes. According to this view, "[p]eople why people often select nondiagnostic overrely on cognitive processes and representations diagnostic information in Bayesian decision
to arrive at their desired conclusions, but situations. Suppose that one must attempt todecide which of two diseases, A or B, a patient
motivation plays a role in determining which ofwith Symptoms X and Y has. One is informed ofthese will be used on a given occasion" (Kunda,
the relative frequency of Symptom X among
1990, p. 480).
people who have Disease A and is then given the
Kunda defended this view, arguing that the choice of obtaining either of the following items
evidence to date is consistent with the assump- of information: the relative frequency of peopletion that motivation affects reasoning, but it with A who have Symptom Y or the relative
does so through cognitive strategies for access- frequency of people with B who have symptom
ing, constructing, and evaluating beliefs:
X. Most people who have been given choices ofthis sort opt for the first; they continue to focus
Although cognitive processes cannot fully account for
the existence of self-serving biases, it appears that theyon the hypothesis that the patient has A, evenplay a major role in producing these biases in that they
though learning the relative frequency of Y
provide the mechanisms through which motivation
given A does not inform the diagnosis, whereas
affects reasoning. Indeed, it is possible that motivation
learning
the relative frequency of X given B
merely provides an initial trigger for the operation ofdoes.
cognitive processes that lead to the desired conclusions,(p. 493)

The primary cognitive operation hypothesized to mediate motivational effects is thebiased searching of memory. Evidence ofvarious types converges, she argued, on the
conclusion that "goals enhance the accessibilityof those knowledge structures—memories, beliefs, and rules—that are consistent with desiredconclusions" (p. 494); "Motivation will cause
bias, but cognitive factors such as the available
beliefs and rules will determine the magnitudeof the bias" (p. 495).
Several of the accounts of confirmation biasthat follow stress the role of cognitive limita-

Assuming a restricted focus on a single
hypothesis, it is easy to see how that hypothesis
might become strengthened even if it is false. An
incorrect hypothesis can be sufficiently close tobeing correct that it receives a considerable
amount of positive reinforcement, which may betaken as further evidence of the correctness ofthe hypothesis in hand and inhibit continued
search for an alternative. In many contexts
intermittent reinforcement suffices to sustain the
behavior that yields it.
People also can increase the likelihood ofgetting information that is consistent withexisting beliefs and decrease the likelihood ofThis document is copyrighted by the American Psychological Association or one of its allied publishers.
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CONFIRMATION BIAS

getting information that is inconsistent withthem by being selective with respect to where
they get information (Frey, 1986). The idea thatpeople tend to expose themselves more to
information sources that share their beliefs than
to those that do not has had considerable
credibility among social psychologists (Festinger, 1957; Klapper, 1960). Sears and Freedman (1967) have challenged the conclusivenessof much of the evidence that has been evoked insupport of this idea. They noted that, when givena choice of information that is supportive of aview one holds and information that is supportive of an opposing view, people sometimes
select the former and sometimes the latter, andsometimes they show no preference. Behavior
in these situations seems to depend on a numberof factors in addition to the polarity of theinformation with respect to one's existing views,
such as people's level of education or social
status and the perceived usefulness of theinformation that is offered.
Sears and Freedman (1967) stopped short,however, of concluding that people are totally
unbiased in this respect. They noted thepossibility that "dramatic selectivity in preferences may not appear at any given moment in
time, but, over a long period, people mayorganize their surroundings in a way that
ensures de facto selectivity" (p. 213). People
tend to associate, on a long-term basis, withpeople who think more or less as they do on
matters important to them; they read authors
with whom they tend to agree, listen to news
commentators who interpret current events in a
way that they like, and so on. The extent towhich people choose their associates because of
their beliefs versus forming their beliefs becauseof their associates is an open question. But it
seems safe to assume that it goes a bit in both
directions. Finding lots of support for one's
beliefs and opinions would be a naturalconsequence of principally associating withpeople with whom one has much in common.
Gilovich (1991) made the important point thatfor many beliefs or expectations confirmatory
events are likely to be more salient thannonconfirmatory ones. If a fortune teller predicts
several events in one's life sometime during theindefinite future, for example, the occurrence ofany predicted event is more likely to remind oneof the original prediction of that event than is its
nonoccurrence. Events predicted by a fortune

199

teller illustrate one-sided events. Unlike twosided events, which have the characteristic that
their nonoccurrence is as noticeable as theiroccurrence, one-sided events are likely to be
noticed while their nonoccurrence is not. (Anexample of a two-sided event would be the tossof a coin following the prediction of a head. In thiscase the nonoccurrence of the predicted outcome would be as noticeable as its occurrence.)
Sometimes decision policies that rule out theoccurrence of certain types of events preclude
the acquisition of information that is counterindicative with respect to a hypothesis. Consider, for example, the hypothesis that only
students who meet certain admission requirements are likely to be successful as college
students. If colleges admit only students whomeet those requirements, a critical subset of thedata that are necessary to falsify the hypothesis(the incidence of students who do not meet theadmission requirements but are neverthelesssuccessful college students) will not exist(Einhorn & Hogarth, 1978). One could argue
that this preclusion may be justified if the
hypothesis is correct, or nearly so, and if thenegative consequences of one type of error(admitting many students who will fail) aremuch greater than those of the other type
(failing to admit a few students who would
succeed). But the argument is circular because it
assumes the validity of the hypothesis inquestion.
Some beliefs are such that obtaining evidence
that they are false is inherently impossible. If I
believe, for example, that most crimes are
discovered sooner or later, my belief may be
reinforced every time a crime is reported by alaw enforcement agency. But by definition,
undiscovered crimes are not discovered, so there
is no way of knowing how many or them there
are. For the same reason, if I believe that mostcrimes go undiscovered, there is no way todemonstrate that this belief is wrong. No matter
how many crimes are discovered, the number of
undiscovered crimes is indeterminable because
being undiscovered means being uncounted.
Some have also given information-processing
accounts of why people, in effect, consider only
the probability of an event, assuming the truth ofa hypothesis of interest, and fail to consider the
probability of the same event, assuming thefalsity of that hypothesis. Evans (1989) argued

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200

RAYMOND S. NICKERSON

that one need not assume the operation of amotivational bias—a strong wish to confirm—in order to account for this failure. It
could signify, according to Evans, a lack ofunderstanding of the fact that, without aknowledge of p(D\~H), p(D\H) gives one nouseful information about p(H\D). That is,p(D\H), by itself, is not diagnostic with respect
to the truth or falsity of H. Possibly people
confuse p(D\H) with p(H\D) and take its
absolute value as an indication of the strength ofthe evidence in favor of H. Bayes's rule ofinverse probability, a formula for getting fromp(D\H) to p(H\D) presumably was motivated,in part, to resolve this confusion.
Another explanation of why people fail toconsider alternatives to a hypothesis in hand isthat they simply do not think to do so. Plausible
alternatives do not come to mind. This is seen bysome investigators to be, at least in part, a matterof inadequate effort, a failure to do a sufficiently
extensive search for possibilities (Baron, 1985,
1994; Kanouse, 1972).

that people show a preference for questions thatwould yield a positive answer if the hypothesis
is correct over questions that would yield anegative answer if the hypothesis is correct
demonstrates that the positive-test strategy is nota simple consequence of the confirmation bias
(Baron et al., 1988; Devine, Hirt, & Gehrke,
1990; Skov & Sherman, 1986).
One could view the results that Wason (1960)
originally obtained with the number-triplet task,
which constituted the point of departure formuch of the subsequent work on confirmationbias, as a manifestation of the positive-test
strategy, according to which one tests cases one
thinks likely to have the hypothesized property(Klayman & Ha, 1987). As already noted, thisstrategy precludes discovering the incorrectnessof a hypothesized rule when instances thatsatisfy the hypothesized rule constitute a subsetof those that satisfy the correct one, but it is aneffective strategy when the instances that satisfy
the correct rule constitute a subset of those thatsatisfy the hypothesized one.
Suppose, for example, that the hypothesizedrule
is successive even numbers and the correct
Positive-Test Strategy or Positivity Bias
one is increasing numbers. All triplets thatArguing that failure to distinguish among satisfy the hypothesized rule also satisfy thedifferent senses of confirmation in the literature correct one, so using only such triplets as test
has contributed to misinterpretations of both cases will not reveal the incorrectness of theempirical findings and theoretical prescriptions, hypothesized rule. But if the hypothesized rule
Klayman and Ha (1987) have suggested that is increasing numbers and the correct rule ismany phenomena of human hypothesis testing successive even numbers, the positive-test stratcan be accounted for by the assumption of a egy—which, in this case means trying various
general positive-test strategy. Application of this triplets of increasing numbers—is likely to
strategy involves testing a hypothesis either by provide the feedback necessary to discover thatconsidering conditions under which the hypoth- the hypothesized rule is wrong. In both of theesized event is expected to occur (to see if it examples considered, the strategy is to selectdoes occur) or by examining known instances of instances for testing that satisfy the hypothits occurrence (to see if the hypothesized esized rule; it is effective in revealing the rule toconditions prevailed). The phenomenon bears be wrong in the second case, not because thesome resemblance to the finding that, in the tester intentionally selects instances that willabsence of compelling evidence one way or the show the rule to be wrong if it is wrong, butother, people are more inclined to assume that a because the relationship between hypothesizedstatement is true than to assume that it is false and correct rules is such that the tester is likely(Clark & Chase, 1972; Gilbert, 1991; Trabasso, to discover the hypothesized rule to be wrong by
Rollins, & Shaughnessy, 1971; Wallsten & selecting test cases intended to show that it isright.
Gonzalez- Vallejo, 1994).
Baron, Beattie, and Hershey (1988), who
Klayman and Ha (1987) analyzed various
referred to the positive-test strategy as the possible relationships between the sets ofcongruence heuristic, have shown that the triplets delimited by hypothesized and correcttendency to use it can be reduced if people are rules in addition to the two cases in which one
asked to consider alternatives, but that they tend set is a proper subset of the other—overlapping
not to consider them spontaneously. The fact sets, disjoint sets, identical sets—and showedThis document is copyrighted by the American Psychological Association or one of its allied publishers.
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CONFIRMATION BIAS

that the likely effectiveness of positive-test
strategy depends on which relationship pertains.
They also analyzed corresponding cases inwhich set membership is probabilistic and drewa similar conclusion. With respect to disconfirmation, Klayman and Ha argued the importanceof distinguishing between two strategies, one
involving examination of instances that are
expected not to have the target property, and theother involving examination of instances that
one expects to falsify, rather than verify, thehypothesis. Klayman and Ha contended that
failure to make this distinction clearly in the pasthas been responsible for some confusion anddebate.
Several investigators have argued that apositive-test strategy should not necessarily beconsidered a biased information-gathering technique because questions prompted by thisstrategy generally do not preclude negative
answers and, therefore, falsification of the
hypothesis being tested (Bassock & Trope,
1984; Hodgins & Zuckerman, 1993; Skov &Sherman, 1986; Trope & Mackie, 1987). It is
important to distinguish, however, between
obtaining information from a test because thetest was intended to yield the information
obtained and obtaining information adventitiously from a test that was intended to yieldsomething other than what it did. Consider again
the number-triplet task. One might select, say,
6-7-8 for either of the following reasons,among others: (a) because one believes the rule
to be increasing numbers and wants a triplet thatfits the rule, or (b) because one believes the rule
to be successive even numbers and wants toincrease one's confidence in this belief by
selecting a triplet that fails to satisfy the rule.
The chooser expects the experimenter's response to the selected triplet to be positive in thefirst case and negative in the second. In either
case, the experimenter's response could be
opposite from what the chooser expects andshow the hypothesized rule to be wrong; theresponse is adventitious because the test yielded
information that the chooser was not seeking
and did not expect to obtain.
The fact that people appear to be more likely
than not to test a hypothesized rule by selecting
cases that they believe will pass muster (that fit
the hypothesized rule) and lend credence to the
hypothesis by doing so justifies describing
typical performance on find-the-rule tasks like201

the triplet task as illustrative of a confirmation
bias. People appear to be much less likely toattempt to get evidence for a hypothesized ruleby choosing a case that they believe does not fit
it (and expecting an informative-negative response) or to select cases with the intended
purpose of ruling out one or more currently
plausible hypotheses from further consideration.
When a test that was expected to yield an
outcome that is positive with respect to ahypothesized rule does not do so, it seems
appropriate to say that the positive-test strategy
has proved to be adventitiously informative.
Clearly, it is possible to select an item that is
consistent with a hypothesized rule for the
purpose of revealing an alternative rule to be
wrong, but there is little evidence that this isoften done. It seems that people generally selecttest items that are consistent with the rule they
believe to be correct and seldom select itemswith falsification in mind.
Klayman and Ha (1987) argued that thepositive-test strategy is sometimes appropriate
and sometimes not, depending on situationalvariables such as the base rates of the phenomena of interest, and that it is effective undercommonly occurring conditions. They argued
too, however, that people tend to rely on it
overly much, treating it as a default strategy to
be used when testing must be done underless-than-ideal conditions, and that it accounts
for many of the phenomena that are generallyinterpreted as evidence of a pervasive confirmation bias.
Evans (1989) has proposed an explanation of
the confirmation bias that is similar in somerespects to Klayman and Ha's (1987) accountand also discounts the possibility that people
intentionally seek to confirm rather than falsify
their hypotheses. Cognitive failure, and not
motivation, he argued, is the basis of thephenomenon:
Subjects confirm, not because they want to, but because
they cannot think of the way to falsify. The cognitive
failure is caused by a form of selective processing
which is very fundamental indeed in cognition—a bias
to think about positive rather than negative information, (p. 42)

With respect to Wason's (1960) results with
the number-triplet task, Evans suggested thatrather than attempting to confirm the hypotheses
they were entertaining, participants may simply
have been unable to think of testing them in aThis document is copyrighted by the American Psychological Association or one of its allied publishers.
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202

RAYMOND S. NICKERSON

negative manner; from a logical point of view,
they should have used potentially disconfirming
test cases, but they failed to think to do so.
Evans (1989) cited the results of several
efforts by experimenters to modify hypothesistesting behavior on derivatives of Wason's
number-set task by instructing participants
about the importance of seeking negative or
disconfirming information. He noted that although behavioral changes were sometimes
induced, more often performance was not
improved. This too was taken as evidence that
results that have been attributed to confirmation
bias do not stem from a desire to confirm, butrather from the difficulty people have in thinking
in explicitly disconfirmatory terms.
Evans used the same argument to account for
the results typically obtained with Wason's(1966, 1968) selection task that have generally
been interpreted as evidence of the operation ofa confirmation bias. Participants select named
items in this task, according to this view,
because these are the only ones that come tomind when they are thinking about what to do.
The bias that is operating is not that of wanting
to confirm a tentative hypothesis but that ofbeing strongly inclined to think only of information that is explicitly provided in the problem
statement. In short, Evans (1989) distinguishedbetween confirmatory behavior, which he acknowledges, and confirmatory intentions, which
he denies. The "demonstrable deficiencies in theway in which people go about testing andeliminating hypotheses," he contended, "are afunction of selective processing induced by awidespread cognitive difficulty in thinking
about any information which is essentiallynegative in its conception" (p. 63). The
tendency to focus on positive information andfail to consider negative information is regarded
not as a conscious cognitive strategy but as theresult of preattentive processes. Positive is notsynonymous with confirmatory, but as moststudies have been designed, looking for positive
cases is tantamount to looking for confirmation.
The idea that people have a tendency to focus
more on positive than on negative information,
like the idea of a confirmation bias, is an old
one. An observation by Francis Bacon (1620/
1939) can again illustrate the point: "It is thepeculiar and perpetual error of the humanunderstanding to be more moved and excited byaffirmatives than negatives; whereas it ought

properly to hold itself indifferently disposed
towards both alike" (p. 36). Evans's (1989)
argument that such a bias can account, at least inpart, for some of the phenomena that are
attributed to a confirmation bias is an intuitively
plausible one. A study in which Perkins et al.,(1983) classified errors of reasoning made in
informal arguments constructed by over 300
people of varying age and educational levelsupports the argument. Many of the errors
Perkins et al. identified involved participants'
failure to consider lines of reasoning that could
be used to defeat or challenge their conclusions.
I will be surprised if a positivity bias turns out
to be adequate to account for all of the ways inwhich what has here been called a confirmation
bias manifests itself, but there are many
evidences that people find it easier to deal withpositive information than with negative: it iseasier to decide the truth or falsity of positive
than of negative sentences (Wason, 1959,1961);
the assertion that something is absent takes
longer to comprehend than the assertion thatsomething is present (Clark, 1974); and inferences from negative premises require more time
to make or evaluate and are more likely to be
erroneous or evaluated incorrectly than are thosethat are based on positive premises (Fodor,
Fodor, & Garrett, 1975). How far the idea can be
pushed is a question for research. I suspect that
failure to try to construct counterarguments or tofind counterevidence is a major and relatively
pervasive weakness of human reasoning.
In any case, the positivity bias itself requires
an explanation. Does such a bias have somefunctional value? Is it typically more importantfor people to be attuned to occurrences than tononoccurrences of possible events? Is language
processed more effectively by one who ispredisposed to hear positive rather than negative
assertions? Is it generally more important to beable to make valid inferences from positive thanfrom negative premises? It would not be
surprising to discover that a positivity bias isadvantageous in certain ways, in which caseBacon's dictum that we should be indifferently
disposed toward positives and negatives would
be wrong.
There is also the possibility that the positivity
bias is, at least in part, motivationally based.
Perhaps it is the case that the processing ofnegative information generally takes more effort
than does the processing of positive information

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CONFIRMATION BIAS

and often we are simply not willing to make theeffort that adequate processing of the negative
information requires. As to why the processingof negative information should require more
effort than the processing of positive information, perhaps it is because positive information
more often than not is provided by the situation,
whereas negative information must be actively
sought, from memory or some other source. It is
generally clear from the statement of a hypothesis, for example, what a positive instance of the
hypothesized event would be, whereas thatwhich constitutes a negative instance may
require some thought.

Conditional Reference Frames
Several investigators have shown that whenpeople are asked to explain or imagine why ahypothesis might be true or why a possible event
might occur, they tend to become more convinced that the hypothesis is true or that theevent will occur, especially if they have notgiven much thought to the hypothesis or event
before being asked to do so (Campbell & Fairey,
1985; Hirt & Sherman, 1985; Sherman et al.,1983). In some cases, people who were asked to
explain why a particular event had occurred andthen were informed that the event did not occur
after they did so, still considered the event more
"likely" than did others who were not asked to
explain why it might occur (Ross et al., 1977).
Koehler (1991), who has reviewed much of
the work on how explanations influence beliefs,
has suggested that producing an explanation is
not the critical factor and that simply coming up
with a focal hypothesis is enough to increase
one's confidence in it. Anything, he suggested,that induces one to accept the truth of ahypothesis temporarily will increase one's
confidence that it is, in fact, true. Calling
attention to a specified hypothesis results in theestablishment of a focal hypothesis, and this inturn induces the adoption of a conditionalreference frame, in which the focal hypothesis is
assumed to be true.
Adoption of a conditional reference frame
influences subsequent hypothesis-evaluation processes in three ways, Koehler (1991) suggested:
it affects the way the problem is perceived, howrelevant evidence is interpreted, and the direction and duration of information search. According to this author,

203

Once a conditional reference frame has been induced
by an explanation task, a certain inertia sets in, whichmakes it more difficult to consider alternative hypotheses impartially. In other words, the initial impression
seems to persist despite the person's efforts to ignore it
while trying to give fair consideration to an alternative
view. (p. 503)

Hoch's (1984) findings regarding people who
were asked to generate reasons for expecting a
specified event and reasons against expecting it
support Koehler's conclusion: in Hoch's study
those who generated the pro reasons first and thecon reasons later considered the event moreprobable than did those who generated the pro
and con reasons in the opposite order. Koehler
related the phenomenon to that of mental set or
fixedness that is sometimes described in discussions of problem solving. He argued that
adopting a conditional reference frame todetermine confidence in a hypothesis is probably a good general method but that, like otherheuristic methods, it also can yield overconfidence in some instances.
In a recent study of gambling behavior,
Gibson, Sanbonmatsu, and Posavac (1997)
found that participants who were asked toestimate the probability that a particular teamwould win the NBA basketball championshipmade higher estimates of that team's probabilityof winning than did control participants who
were not asked to focus on a single team; thefocused participants also were more willing to
bet on the focal team. This suggests that
focusing on one among several possible event
outcomes, even as a consequence of being
arbitrarily forced to do so, can have the effect ofincreasing the subjective likelihood of thatoutcome.

Pragmatism and Error Avoidance
Much of the discussion of confirmation bias ispredicated on the assumption that, in thesituations in which it has generally been
observed, people have been interested in determining the truth or falsity of some hypothesises) under consideration. But determining its
truth or falsity is not the only, or necessarily
even the primary, objective one might have withrespect to a hypothesis. Another possibility isthat of guarding against the making of certain
types of mistakes.
In many real-life situations involving theevaluation of a meaningful hypothesis, or theThis document is copyrighted by the American Psychological Association or one of its allied publishers.
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204

RAYMOND S. NICKERSON

making of a choice with an outcome that reallymatters to the one making it, some ways of
being wrong are likely to be more regretable
than others. Investigators have noted that thisfact makes certain types of biases functional inspecific situations (Cosmides, 1989; Friedrich,
1993; Hogarth, 1981; Schwartz, 1982). When,for example, the undesirable consequences of
judging a true hypothesis to be false are greaterthan those of judging a false hypothesis to betrue, a bias toward confirmation is dictated bysome normative models of reasoning and bycommon sense.
Friedrich (1993) argued that "our inference
processes are first and foremost pragmatic,
survival mechanisms and only secondarily truth
detection strategies" (p. 298). In this view,peoples' inferential strategies are well suited tothe identification of potential rewards and the
avoidance of costly errors, but not to theobjective of hypothesis testing in accordancewith the logic of science. Inference strategies
that are often considered to be seriously flawed
not only may have desired effects in real-worldcontexts, but may also be seen as correct whenjudged in terms of an appropriate standard.
To illustrate the point, Friedrich (1993) usedthe example of an employer who wants to test
the hunch that extroverts make the best salespeople. If the employer checked the sales
performance only of extroverts, found it to bevery good and, on this basis, decided to hire only
extroverts for sales positions, one would say that
she had not made an adequate test of her hunch
because she did not rule out the possibility that
introverts might do well at sales also. But if her
main objective was to avoid hiring people whowill turn out to be poor at sales, satisfying
herself that extroverts make good sales people
suffices; the fact that she has not discovered that
introverts can be good at sales too could meanthat she will miss some opportunities by nothiring them, but it does not invalidate thedecision to hire extroverts if the objective is toensure that poor performers do not get hired.
Schwartz (1982) also argued that when
responses have consequences that really matter,
people are more likely to be concerned about
producing desirable outcomes than about determining the truth or falsity of hypotheses.
Contingent reinforcement may create functional
behavioral units that people tend to repeat
because the behaviors have worked in the past,

and a bias toward confirmation is one of thestereotyped forms of behavior in which theoperation of these units manifests itself. In
performing a rule-discovery task, one may be
attempting to maximize the probability ofgetting a positive response, which is tantamountto seeking a rule that is sufficient but notnecessary to do so. Given that one has identified
a condition that is sufficient for producing
desired outcomes, there may be no compelling
reason to attempt to determine whether thatcondition is also necessary. Schwartz pointed
out too that, especially in social situations suchas some of those studied by Snyder andcolleagues attempting to evaluate hypotheses byfalsification would require manipulating people
and could involve considerable social cost.
Baron (1994) noted that truth seeking or
hypothesis testing often may be combined with
one or more other goals, and that one's behavior
then also must be interpreted in the light of theother goal(s). If, for example, one is curious as
to why a cake turned out well despite the factthat certain ingredients were substituted for
those called for by the recipe, one may bemotivated to explore the effects of the substitutions in such a way that the next experimental
cake is likely to turn out well too.
When using a truth-seeking strategy would
require taking a perceived risk, survival is likely
to take precedence over truth finding, and it ishard to argue that rationality would dictate
otherwise. It would seem odd to consider
irrational the refusal, say, to eat mushrooms thatone suspected of being poison because thedecision is calculated to preserve one's wellbeing rather than to shed light on the question ofwhether the suspicion is indeed true. In general,
the objective of avoiding disastrous errors may
be more conducive to survival than is that oftruth determination.
The desire to avoid a specific type of error
may coincidentally dictate the same behavior as
would the intention to determine the truth or
falsity of a hypothesis. When this is the case, thebehavior itself does not reveal whether theindividual's intention is to avoid the error or totest the hypothesis. Friedrich (1993) suggested
that some behavior that has been taken as
evidence of people's preference for normative
diagnostic tests of hypotheses and their interestin accuracy could have been motivated insteadby the desire to avoid specific types of errors. In

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CONFIRMATION BIAS

other words, even when behavior is consistent
with the assumption of truth seeking, it sometimes may be equally well interpreted, according to this view, in terms of error-minimizing
strategies.
The assumption that decisions made orconclusions drawn in many real-life situations
are motivated more by a desire to accomplish
specific practical goals or to avoid certain typesof errors than by the objective of determining
the truth or falsity of hypotheses is a plausible
one. Pragmatic considerations of this sort could
often lead one to accept a hypothesis as true—tobehave as though it were true—on less thancompelling evidence that it is so, thus constituting a confirmation bias of sorts.

Educational Effects
At all levels of education, stress is placed on
the importance of being able to justify what one
believes. I do not mean to question theappropriateness of this stress, but I do want tonote the possibility that, depending on how it isconveyed, it can strengthen a tendency to seek
confirming evidence selectively or establish
such a tendency if it does not already exist. If
one is constantly urged to present reasons for
opinions that one holds and is not encouraged
also to articulate reasons that could be given
against them, one is being trained to exercise aconfirmation bias.
Narveson (1980) noted that when students
write compositions, they typically evaluate theirclaims by considering supporting evidence only.
He argued that standard methods for teachingcomposition foster this. The extent to which theeducational process makes explicit the distinction between case-building and evidenceweighing deserves more attention. If the distinction is not made and what is actually casebuilding passes for the impartial use of evidence,
this could go some way toward accounting forthe pervasiveness and strength of the confirmation bias among educated adults.
Ideally, one would like students, and people
in general, to evaluate evidence objectively andimpartially in the formation and evaluation of
hypotheses. If, however, there are fairly pervasive tendencies to seek or give undue weight toevidence that is confirmatory with respect tohypotheses that people already hold and to avoid
or discount evidence that is disconfirmatory

205

with respect to them, there is a need to be
especially sensitive to the educational practices
that could serve to strengthen an already strong
bias.
Utility of Confirmation Bias
Most commentators, by far, have seen theconfirmation bias as a human failing, a tendency
that is at once pervasive and irrational. It is notdifficult to make a case for this position. The
bias can contribute to delusions of many sorts, tothe development and survival of superstitions,and to a variety of undesirable states of mind,
including paranoia and depression. It can be
exploited to great advantage by seers, soothsayers, fortune tellers, and indeed anyone with aninclination to press unsubstantiated claims. One
can also imagine it playing a significant role in
the perpetuation of animosities and strife
between people with conflicting views of theworld.
Even if one accepts the idea that theconfirmation bias is rooted more in cognitive
limitations than in motivation, can anyone doubtthat whenever one finds oneself engaged in averbal dispute it becomes very strong indeed? In
the heat of an argument people are seldom
motivated to consider objectively whateverevidence can be brought to bear on the issue
under contention. One's aim is to win and theway to do that is to make the strongest possible
case for one's own position while countering,
discounting, or simply ignoring any evidence
that might be brought against it. And what is trueof one disputant is generally true of the other,which is why so few disputes are clearly won or
lost. The more likely outcome is the claim of
victory by each party and an accusation ofrecalcitrance on the part of one's opponent.
But whenever an apparently dysfunctional
trait or behavior pattern is discovered to bepervasive, the question arises as to how, if it isreally dysfunctional, did it get to be so
widespread. By definition, dysfunctional tendencies should be more prone to extinction thanfunctional ones. But aspects of reasoning thatare viewed as flawed from one perspective
sometimes can be considered appropriate, perhaps because they are adaptively useful incertain real-world situations from another perspective (Arkes, 1991; Funder, 1987; Greenwald, 1980). Does the confirmation bias have

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RAYMOND S. NICKERSON

some adaptive value? Does it serve some useful
purpose(s)?

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Utility in Science
According to the principle of falsifiability(Popper, 1959), an explanation (theory, model,
hypothesis) cannot qualify as scientific unless it
is falsifiable in principle. This is to say there
must be a way to show the explanation to befalse if in fact it is false. Popper focused on
falsifiability as the distinguishing characteristicof the scientific method because of a convictionthat certain theories of the day (in particular,
Marx's theory of history, Freud's theory ofpsychoanalysis, and Adler's individual psychology) "appeared to be able to explain practicallyeverything that happened within the fields towhich they referred" (Popper, 1962/1981, p.
94). Thus subscribers to any one of these
theories were likely to see confirming evidence
anywhere they looked. According to Popper(1959), the most characteristic element in thissituation seemed to be the incessant stream ofconfirmations, of observations which 'verified'
the theories in question; and this point wasconstantly emphasized by their adherents" (p.
94). There was, in Popper's view, no conceivable evidence that could be brought to bear on
any of these theories that would be viewed by anadherent as grounds for judging it to be false.
Einstein's theory of relativity impressed Popper
as being qualitatively different in the important
respect that it made risky predictions, which is
to say predictions that were incompatible withspecific possible results of observation. This
theory was, in principle, refutable by empiricalmeans and therefore qualified as scientificaccording to Popper's criterion.
Although Popper articulated the principle offalsifiability more completely than anyone
before him, the idea has many antecedents inphilosophy and science. It is foreshadowed, forexample, in the Socratic method of refutation(elenchos), according to which what is to betaken as truth is whatever survives relentlessefforts at refutation (Maclntyre, 1988). Lakatos(1976, 1978) and Polya (1954a, 1954b) havediscussed the importance of this attitude inreasoning in mathematics—especially in proofmaking—at length. The principle of falsifiability was also anticipated by T. H. Huxley(1894/1908), who spoke of "a beautiful hypoth-esis killed by an ugly fact," and by David
Hartley (1748/1981), who proposed a rule offalse. According to this rule, the acceptability of
any supposition or hypothesis should be its
ability to provide the basis for the deduction ofobservable phenomena: "He that forms hypotheses from the first, and tries them by the facts,soon rejects the most unlikely ones; and being
freed from these, is better qualified for theexamination of those that are probable" (p. 90).
Hypotheses are strengthened more whenhighly competent scientists make concerted
efforts to disprove them and fail than whenefforts at disproof are made by less competent
investigators or in are made half-hearted ways.
As Polya (1954a) put it, "the more danger, themore honor." What better support could Einstein's corpuscular theory of light have received
than Millikan's failure to show it to be wrong,
despite 10 years of experimentation aimed atdoing so? (It is interesting to note thatthroughout this period of experimentation,
Millikan continued to insist on the untenabilityof the theory despite his inability to show by
experiment its predictions to be in error.)
Despite the acceptance of the falsifiability
principle by the scientific community as awhole, one would look long and hard to find an
example of a well-established theory that was
discarded when the first bit of disconfirming
evidence came to light. Typically an establishedtheory has been discarded only after a better
theory has been offered to replace it. Perhaps
this should not be surprising. What astronomer,
Waismann (1952) asked, would abandon Kepler's laws on the strength of a single observation? Scientists have not discarded the idea thatlight travels at a constant speed of about 300,000
kilometers per second and that nothing can
travel faster simply because of the discovery ofradio sources that seem to be emanating from asingle quasar and moving away from each otherat more than nine times that speed (Gardner,1976). It appears that, the principle of falsifiability notwithstanding, "Science proceeds on
preponderance of evidence, not on finality"(Drake, 1980, p. 55).
Application of the falsifiability principle to
the work of individual scientists seems toindicate that when one comes up with a new
hypothesis, one should immediately try tofalsify it. Common sense suggests this too; if the
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CONFIRMATION BIAS

the less time one will waste entertaining it. In
fact, as has already been noted, there is little
evidence that scientists work this way. To thecontrary, they often look much harder for
evidence that is supportive of a hypothesis thanfor evidence that would show it to be false.
Kepler's laborious effort to find a connection
between the perfect polyhedra and the planetary
orbits is a striking example of a search by ascientist for evidence to confirm a favoredhypothesis. Here is his account of the connection he finally worked out and his elation upon
finding it, as quoted in Boorstin (1985):
The earth's orbit is the measure of all things;
circumscribe around it a dodecahedron, and the circle
containing this will be Mars; circumscribe around Marsa tetrahedron, and the circle containing this will be
Jupiter; circumscribe around Jupiter a cube, and thecircle containing this will be Saturn. Now inscribe
within the earth an icosahedron, and the circle
contained in it will be Mercury. You now have thereason for the number of planets.... This was theoccasion and success of my labors. And how intense
was my pleasure from this discovery can never be
expressed in words. I no longer regretted the time
wasted. Day and night I was consumed by thecomputing, to see whether this idea would agree with
the Copernican orbits, or if my joy would be carriedaway by the wind. Within a few days everythingworked, and I watched as one body after another fit
precisely into its place among the planets, (p. 310)

People are inclined to make light of thisparticular accomplishment of Kepler's today,
but it was a remarkable intellectual feat. Theenergy with which he pursued what he saw as an
intriguing clue to how the world works is
inspiring. Bell (1946/1991) argued that it was
Kepler's "Pythagorean faith in a numericalharmony of the universe" that sustained him "inhis darkest hours of poverty, domestic tragedy,
persecution, and twenty-two years of discouragement as he calculated, calculated, calculated to
discover the laws of planetary orbits" (p. 181).
The same commitment that kept Kepler inpursuit of confirmation of his polyhedral modelof planetary orbits yielded the three exquisitly
beautiful—and correct—laws of planetary motion for which he is honored today.
My point is not to defend the confirmation
bias as an effective guide to truth or even as aheuristically practical principle of logical thinking. It is simply to note that the quest to findsupport for a particular idea is a common
phenomenon in science (I. B. Cohen, 1985;
Holton, 1973), that such a quest has often

207

provided the motivation to keep scientists
working on demanding intellectual problems
against considerable odds, and that the resulting
work has sometimes yielded lasting, if unexpected, results.
Students of the scientific process have notedthe conservatism of science as an institution(I. B. Cohen, 1985; T. S. Kuhn, 1970), andillustrations of it were given in an earlier part ofthis article. This conservatism can be seen as an
institutional confirmation bias of sorts. Should
we view such a bias as beneficial overall or
detrimental to the enterprise? An extensive
discussion of this question is beyond the scopeof this article. However, it can be argued that a
degree of conservativism plays a stabilizing rolein science and guards the field against uncriticalacceptance of so-called discoveries that fail tostand the test of time.
Price (1963) referred to conservatism in the
body of science as "a natural counterpart to the
open-minded creativity that floods it with too
many new ideas" (p. 64). Justification for acertain degree of conservatism is found in theembarrassment that the scientific community
has occasionally experienced as a consequenceof not being sufficiently sceptical of new
discoveries. The discovery of magnetic monopoles, which was widely publicized before aclose examination of the evidence forced moreguarded interpretations, and that of polywater,which motivated hundreds of research projects
over decade following its discovery in the
1960s, are examples.
The scientific community's peremtory rejection of Wegener's (1915/1966) theory of continental drift when it was first put forth is often
held out as an especially egregious example ofexcessive—and self-serving—conservatism on
the part of scientists. The view has also been
expressed, however, that the geologists who
dismissed Wegener's theory, whatever theirmotivations, acted in a way that was conduciveto scientific success. Solomon (1992), who took
this position, acknowledged that the behavior
was biased and motivated by the desire toprotect existing beliefs, but she argued that"bias and belief perseverance made possible the
distribution of effort, and this in turn led to theadvancement of the debate over [continental]
drift" (p. 443).
Solomon's (1992) review of this chapter inthe history of geological research makes it clear

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208

RAYMOND S. NICKERSON

that the situation was not quite as simple assome accounts that focus on the closedmindedness of the geologists at the time would
lead one to believe. The idea of continental drift
was not entirely new with Wegener, forexample, although Wegener was the first to
propose a well-developed theory. A serious
limitation of the theory was its failure to identify
a force of sufficient magnitude to account for the
hypothesized movement of continents. (Thenotion of plate tectonics and evidence regarding
sea-floor spreading came much later; LeGrand,
1988.)
Solomon (1992) argued that it was, in part,
because Wegener was not trained as a geologist
and therefore not steeped in the "stabilist"
theories of the time, that his thinking, relatively
unconstrained by prior beliefs about the stabilityof continents, could easily embrace a possibility
that was so contrary to the prevailing view. She
pointed out too that when geologists began toaccept the notion of drift, as evidence favoring it
accumulated, it was those with low publication
rates who were the most likely to do so: "theirbeliefs were less entrenched (cognitively speaking) than those who had reasoned more andproduced more, so belief perseverance was lessof an impediment to acceptance of drift" (p. 449).
Although Solomon (1992) argued that bias
and belief perseverance were responsible formuch of the distribution of research effort thatled finally to the general acceptance of thetheory of continental drift, and to that of plate
tectonics to which it led in turn, she does notclaim that a productive distribution could nothave been effected without the operation ofthese factors. The question of whether thesefactors facilitated or impeded progress ingeology remains an unanswered one; it is notinconceivable that progress could have been
faster if the distribution of effort were determined on some other basis.
In any case, it can be argued that a certain
degree of conservativism serves a useful stabilizing role in science and is consistent with, if notdictated by, the importance science attaches totestability and empirical validation. Moreover, if
Ziman (1978) is right, the vast majority of the
new hypotheses put forward by scientists prove
to be wrong:
Even in physics, there is no infallible procedure forgenerating reliable knowledge. The calm order andperfection of well-established theories, accredited byinnumerable items of evidence from a thousand
different hands, eyes and brains, is not characteristic of
the front-line of research, where controversy, conjecture, contradiction and confusion are rife. The physics
of undergraduate test-books is 90% true; the contents ofthe primary research journals of physics is 90% false,(p. 40)"According to temperament," Ziman noted,"one may be impressed by the coherence ofwell-established theories, or horrified by thecontradictions of knowledge in the making" (p.
100).
Fischhoff and Beyth-Marom (1983) made a
related point when commenting on Mahoney's(1977) finding that scientists tended to be lesscritical of a fictitious study that reported results
supportive of the dominant hypothesis in theirfield than of one that reported results that wereinconsistent with it. They noted that a reluctanceby scientists to relinquish pet beliefs is only one
interpretation that could be put on the finding.
Another possibility is that what appears to bebiased behavior reflects "a belief that investigators who report disconfirming results tend to use
inferior research methods (e.g., small samples
leading to more spurious results), to commit
common mistakes in experimental design, or,simply, to be charlatans" (Fischoff and BeythMarom, 1983, p. 251). To the extent that such abelief is accurate, a bias against the readyacceptance of results that are disconfirming ofprevailing hypotheses can be seen as a safeguard
against precipitous changes of view that may
prove to be unjustified. It does not follow, ofcourse, that this is the only reason for a
confirmation bias in science or the only effect.

Is Belief Perseverance Always Bad?
It is easy to see both how the confirmation
bias helps preserve existing beliefs, whether trueor false, and how the perseverance of unjustified
beliefs can cause serious problems. Is there
anything favorable to be said about a bias thattends to perpetuate beliefs independently of
their factuality? Perhaps, at least from thenarrow perspective of an individual's mental
health. It may help, for example, to protect one's
ego by making one's favored beliefs lessvulnerable than they otherwise would be.
Indeed, it seems likely that a major reason why
the confirmation bias is so ubiquitous and soenduring is its effectiveness in preserving

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CONFIRMATION BIAS

preferred beliefs and opinions (Greenwald,1980).
But even among people who might see somebenefit to the individual in a confirmation bias,
probably few would contest the claim that when
the tendency to persevere in a belief is so strongthat one refuses to consider evidence that does
not support that belief, it is irrational and offends
our sense of intellectual honesty. That is not to
say that dogmatic confidence in one's own
beliefs and intolerance of opposing views cannever work to one's advantage. Boorstin (1958)
argued, for example, that it was precisely thesequalities that permitted the 17th-century New
England Puritans to establish a society with theingredients necessary for survival and prosperity. He wrote,
Had they spent as much of their energy in debating witheach other as did their English contemporaries, theymight have lacked the single-mindedness needed toovercome the dark, unpredictable perils of a wilderness. They might have merited praise as precursors ofmodern liberalism, but they might never have helped
found a nation, (p. 9)

Contrary to the popular stereotype of the
Puritans, they were not preoccupied withreligious dogma but rather with more practicalmatters because, as Boorstin noted, they had no
doubts and allowed no dissent. They worried
about such problems as how to select leaders
and representatives, the way to establish theproper limits of political power, and how toconstruct a feasible federal organization.
The question of the conditions under which
one should retain, reject, or modify an existing
belief is a controversial one (Cherniak, 1986;
Harman, 1986; Lycan, 1988). The controversy is
not likely to be settled soon. Whatever theanswer to the question is, the confirmation bias
must be recognized as a major force that works
against easy and frequent opinion change.
Probably very few people would be willing togive up long-held and valued beliefs on the first
bit of contrary evidence found. It is natural to bebiased in favor of one's established beliefs.
Whether it is rational is a complicated issue that
can too easily be treated simplistically; however,the view that a person should be sufficiently
objective and open minded to be willing to tossout any belief upon the first bit of evidence that
it is false seems to me wrong for several reasons.
Many, perhaps most, of the beliefs that matterto individuals tend not to be the type that can be209

falsified, in the Popperian sense, by a single
counterindicative bit of data. They tend rather tobe beliefs for which both supportive andcounterindicative evidence can be found, andthe decision as to whether to hold them isappropriately made on the basis of the relative
weights or merits of the pro and con arguments.
Second, it is possible to hold a belief for goodand valid reasons without being able to produce
all of those reasons on demand. Some beliefs areshaped over many years, and the fact that one
cannot articulate every reason one has or has
ever had for a particular one of them does notmean that it is unfounded. Also, as Nisbett andRoss (1980) pointed out, there are practical time
constraints that often limit the amount ofprocessing of new information one can do. Inview of these limitations, the tendency topersevere may be a stabilizing hedge against
overly frequent changes of view that would
result if one were obliged to hold only beliefs
that one could justify explicitly at a moment's
notice. This argument is not unlike the one
advanced by Blackstone (1769/1962) in defenseof not lightly scuttling legal traditions.
Third, for assertions of the type that represent
basic beliefs, there often are two ways to be
wrong: to believe false ones, or to disbelieve
true ones. For many beliefs that people hold,these two possibilities are not equally acceptable, which is to say that an individual might
consider it more important to avoid one type oferror than the other. This is, of course, theargument behind Pascal's famous wager.
To argue that it is not necessarily irrational torefuse to abandon a long-held belief upon
encountering some evidence that appears contradictory is not to deny that there is such a thing as
holding on to cherished beliefs too tenaciously
and refusing to give a fair consideration tocounterindicative evidence. The line between
understandable conservativism with respect tochanging established beliefs and obstinate closedmindedness is not an easy one to draw. But
clearly, people sometimes persevere beyond
reason. Findings such as those of Pitz (1969),
Pitz et al. (1967), Lord et al. (1979), andespecially Ross et al. (1975), who showed thatpeople sometimes persevere in beliefs evenwhen the evidence on which the beliefs were
initially formed has been demonstrated to them
to be fraudulent, have provided strong evidenceof that fact.

RAYMOND S. NICKERSON

210

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Confirmation Bias Compounding
Inadequate Search
The idea that inadequate effort is a basic
cause of faulty reasoning is common among
psychologists. Kanouse (1972) suggested thatpeople may be satisfied to have an explanation
for an event that is sufficient and not feel theneed to seek the best of all possibilities. Nisbett
and Ross (1980) supported the same idea andsuggested that it is especially the case whenattempting to come up with a causal explanation:
The lay scientist seems to search only until a plausible
antecedent is discovered that can be linked to theoutcome through some theory in the repertoire. Given
the richness and diversity of that repertoire, such asearch generally will be concluded quickly and easily.
A kind of vicious cycle results. The subjective ease of
explanation encourages confidence, and confidence
makes the lay scientist stop searching as soon as aplausible explanation is adduced, so that the complexities of the task, and the possibilities for "alternative
explanations" no less plausible than the first, are neverallowed to shake the lay scientist's confidence, (p. 119,120)

Perkins and his colleagues expressed essentially the same idea with their characterizationof people as make-sense epistimologists (Perkins et al., 1983, 1991). The idea here is thatpeople think about a situation only to the extent
necessary to make sense—perhaps superficial
sense—of it:
When sense is achieved, there is no need to continue.
Indeed, because further examination of an issue might
produce contrary evidence and diminish or cloud thesense of one's first pass, there is probably reinforcement for early closure to reduce the possibility ofcognitive dissonance. Such a makes-sense approach isquick, easy, and, for many purposes, perfectly adequate. (Perkins et al., 1991, p. 99)

Baron (1985, 1994) and Pyszczynski andGreenberg (1987) also emphasized insufficient
search as the primary reason for the prematuredrawing of conclusions.
All of these characterizations of the tendencyof people to do a less-than-thorough searchthrough the possibilities before drawing conclusions or settling on causal explanations are
consistent with Simon's (1957,1983/1990) viewof humans as satisficers, as opposed to optimizers or maximizers. The question of interest in
the present context is that of how the criterion
for being satisfied should be set. Given thatsearch is seldom exhaustive and assuming that,in many cases, it cannot be, how much should beenough? How should one decide when to stop?
Without at least tentative answers to thesetypes of questions, it is difficult to say whether
any particular stopping rule should be considered rational. Despite this vagueness withrespect to criteria, I believe that the prevailing
opinion among investigators of reasoning is thatpeople often stop—come to conclusions, adopt
explanations—before they should, which is not
to deny that they may terminate a search more
quickly when time is at a premium and searchlonger when accuracy is critical (Kruglanski,
1980; Kruglanski & Ajzen, 1983; Kruglanski &Freund, 1983). The search seems to be not only
less than extensive but, in many cases, minimal,
stopping at the first plausible endpoint.
If this view is correct, the operation of aconfirmation bias will compound the problem.
Having once arrived at a conclusion, belief, orpoint of view, however prematurely, one may
thereafter seek evidence to support that position
and interpret newly acquired information in a
way that is partial to it, thereby strengthening it.
Instead of making an effort to test an initial
hypothesis against whatever counterindicative
evidence might be marshalled against it, one
may selectively focus on what can be said in its
favor. As the evidence favoring the hypothesis
mounts, as it is bound to do if one gives
credence to evidence that is favorable andignores or discounts that that is not, one will
become increasingly convinced of the correctness of the belief one originally formed.
Concluding Comments
We are sometimes admonished to be tolerantof the beliefs or opinions of others and critical of
our own. Laplace (1814/1956), for example,
gave this eloquent advice:
What indulgence should we not have . . . for opinions
different from ours, when this difference often depends
only upon the various points of view where circumstances have placed us! Let us enlighten those whom
we judge insufficiently instructed; but first let us
examine critically our own opinions and weigh with
impartiality their respective probabilities, (p. 1328)

But can we assess the merits of our own
opinions impartially? Is it possible to put abelief that one holds in the balance with an
opposing belief that one does not hold and givethem a fair weighing? I doubt that it is. But that

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CONFIRMATION BIAS

is not to say that we cannot hope to learn to do
better than we typically do in this regard.
In the aggregate, the evidence seems to me
fairly compelling that people do not naturally
adopt a falsifying strategy of hypothesis testing.
Our natural tendency seems to be to look forevidence that is directly supportive of hypotheses we favor and even, in some instances, of
those we are entertaining but about which are
indifferent. We may look for evidence that isembarrassing to hypotheses we disbelieve or
especially dislike, but this can be seen as
looking for evidence that is supportive of thecomplementary hypotheses. The point is that we
seldom seem to seek evidence naturally thatwould show a hypothesis to be wrong and to doso because we understand this to be an effective
way to show it to be right if it really is right.
The question of the extent to which theconfirmation bias can be modified by training
deserves more research than it has received.
Inasmuch as a critical step in dealing with any
type of bias is recognizing its existence, perhaps
simply being aware of the confirmation bias—of
its pervasiveness and of the many guises inwhich it appears—might help one both to be alittle cautious about making up one's mind
quickly on important issues and to be somewhat
more open to opinions that differ from one's
own than one might otherwise be.
Understanding that people have a tendency tooverestimate the probable accuracy of their
judgments and that this tendency is due, at leastin part, to a failure to consider reasons why thesejudgments might be inaccurate provides arationale for attempting to think of reasons forand (especially) against a judgment that is to be
made. Evidence that the appropriateness ofpeople's confidence in their judgments can be
improved as a consequence of such efforts is
encouraging (Arkes, Faust, Guilmette, & Hart,
1988; Hoch, 1984,1985; Koriatetal., 1980).
The knowledge that people typically consider
only one hypothesis at a time and often make theassumption at the outset that that hypothesis istrue leads to the conjecture that reasoning might
be improved by training people to think ofalternative hypotheses early in the hypothesisevaluation process. They could be encouraged
to attempt to identify reasons why the complement of the hypothesis in hand might be true.
Again, the evidence provides reason for optimism that the approach can work (C. A.

Anderson, 1982; C. A. Anderson & Sechler,
1986; Lord, Lepper, & Preston, 1984).
To the extent that what appear to be biases are
sometimes the results of efforts to avoid certain
types of decision errors (Friedrich, 1993),
making these other types of possible errors moresalient may have a debiasing effect. On the other
hand, if the errors that one is trying to avoid are,in fact, more costly than bias errors, suchdebiasing might not be desirable in all instances.
Here the distinction between the objective of
determining the truth or falsity of a hypothesis
and that of avoiding an undesirable error (at theexpense of accepting the possibility of committing a less undesirable one) is an important one
to keep in mind.
Finally, I have argued that the confirmation
bias is pervasive and strong and have reviewed
evidence that I believe supports this claim. The
possibility will surely occur to the thoughtful
reader that what I have done is itself an
illustration of the confirmation bias at work. I
can hardly rule the possibility out; to do sowould be to deny the validity of what I am
claiming to be a general rule.
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Received August 1, 1997
Revision received December 16, 1997
Accepted December 18, 1997