Journal of Economic Perspectives—Volume 22, Number 3—Summer 2008 —Pages 173–188

Probability and Uncertainty in
Economic Modeling
Itzhak Gilboa, Andrew W. Postlewaite, andDavid Schmeidler

S

ince the early days of probability theory, there has been a distinction
between probabilities that are given, as in a game of chance, and probabilities that are not given, but reflect a subjective degree of belief; Hacking(1975) and Shafer (1978) offer historical surveys. In economics, Knight (1921) is
typically credited with the distinction between situations of “risk” and of “uncertainty.” In his formulation, “risk” designates situations in which probabilities are
known, or knowable in the sense that they can be estimated from past data andcalculated using the laws of probability. By contrast, “uncertainty” refers to situations in which probabilities are neither known, nor can they be deduced, calculated, or estimated in an objective way. The Bayesian approach minimizes theimportance of this distinction by introducing the notion of “subjective probability.”
According to this approach, when objective probabilities are not known, they canbe replaced by subjective ones, so that problems of decision under uncertainty are
reduced to problems of decision under risk.
The standard practice in economics when modeling situations of uncertainty
is to follow the Bayesian approach and to assume that people have probabilistic
beliefs over any source of uncertainty, that they update these beliefs in accordancewith Bayes’s rule, and that they use these probabilistic beliefs in decision making,y Itzhak Gilboa is Professor of Economics, Tel-Aviv University, Tel Aviv, Israel, andProfessor of Finance and Economics, HEC, Paris, France. Andrew Postlewaite is Harry P.
Kamen Professor of Economics and Professor of Finance, University of Pennsylvania,
Philadelphia, Pennsylvania. David Schmeidler is Professor at the School of Mathematical
Sciences and at the School of Business Administration, Tel Aviv University, Tel Aviv,
Israel, and Professor of Economics, The Ohio State University, Columbus, Ohio. Theire-mail addresses are 具tzachigilboa@gmail.com典, 具apostlew@econ.upenn.edu典, and具schmeid@tau.ac.il典, respectively.

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typically as a basis for expected utility maximization. This paradigm is an elegant
and coherent way to deal with uncertainty. Yet, it is not always clear how subjective
beliefs should be formed. The following two examples illustrate the problem.
Example 1: Ann is an admissions officer for an economics graduate program.
Every year she reviews a large number of applications to assess the probability thatvarious candidates are likely to succeed in the program if admitted. Faced with aparticular file, how should she assign a probability to the candidate’s success?
Example 2: Bob is a high school graduate considering joining the military. The
advantages of a military career are relatively clear to him. The possible costs,
however, are subject to uncertainty. Specifically, Bob realizes that if the UnitedStates engages in a war in the next few years, he is likely to be stationed overseas,
be involved in combat, and risk his life. To make a rational decision, Bob attempts
to assess the probability of such a war.
Both Ann and Bob would prefer to have probabilistic assessments that are“objective” or even “scientific.” However, there are no agreed-upon methods for
assigning probabilities to the events that concern them. Laplace’s “principle ofindifference”—also known as the “principle of insufficient reason”—suggests assigning equal probabilities to all possible eventualities, say, a 50 –50 chance of
success vs. failure, or of war vs. peace. This approach is clearly not suitable in these
problems, where sufficient information is available to break the symmetry between
the two possible outcomes. Indeed, Ann and Bob should know better than to assign
a default of 50 percent to each outcome.
However, the existing empirical evidence is of limited use. When Ann considers a particular candidate, she does not wish to rely on the overall percentage ofstudents who have succeeded, because these students differ in a variety of variables,and some students’ performance appears more relevant than others. It would benatural for Ann to run a regression using observable characteristics of past applicants on some measure of their success in the program. But quantifying thecharacteristics may leave much to Ann’s intuition. Moreover, the choice of thevariables for the regression model as well the details of the statistical procedure
introduce a certain degree of subjectivity.
Bob’s belief formation problem is even more difficult than Ann’s. He might
attempt to employ an empirical frequency approach for his problem. He would
then consider a database consisting of conflict situations and calculate the percentage of these situations that resulted in war. This relative frequency might be taken
as a proxy for the probability of a war occurring in the next few years. But Bob will
be considering a database of conflict situations that differ from each other in avariety of variables. Some of them will be recent; others will not. Some will involve
similar countries, and some won’t. Taking all recorded conflicts into account would
be unreasonable, but considering only “identical” situations would result in an
empty comparison set. Moreover, regression analysis seems even more problematicin Bob’s case than in Ann’s. The resolutions of different conflicts throughout
history are causally interdependent in intricate ways that are not precisely known
and cannot be captured by simple regression models. Thus, neither empirical

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frequencies nor regression analysis offer a clear procedure for the generation ofbeliefs.
In many economic problems of interest it is not clear how one should define
probabilities. Probabilities are actually “given” only in very restricted situations suchas state lotteries or casino games. In other situations, such as insurance problems,
probabilities can be reasonably approximated by relative frequencies of comparable instances computed from publicly available data. But in a vast range of economic problems, probabilities are neither explicitly given nor can they be approximated by relative frequencies or regression analysis. The goal of this paper is to ask
how probabilities should be defined in these situations, and if probabilities cannot
be defined in a satisfactory way, how beliefs should be modeled.
We begin with a discussion of subjective probability, which is the standard
approach to problems involving uncertainty and which relies on well-known axiomatic foundations. We will argue, however, that the axioms underlying subjective
probability are in some ways too restrictive, and in other ways too general: on the
one hand, we hold that rational decisionmakers may violate the axioms, while, on
the other hand, that the axioms do not restrict probabilistic beliefs in any way.1
With this background in place, we consider the implications of these limitations of
the Bayesian approach to economic modeling. On the one hand, because theBayesian model is too restrictive, one may wish to consider alternative models. We
discuss alternative models that account for a wider set of phenomena than theBayesian one and offer a few examples of economic models that make use of suchapproaches. On the other hand, because the Bayesian model does not say enoughabout the generation of beliefs, we point to a research agenda that will develop
formal, explicit theories of the belief formation process. Such theories may help
both to refine the predictions of economic models when agents are Bayesian andto improve these predictions when they are not.

Subjective Probabilities
In what has become the classical theory of consumer decision making, von
Neumann and Morgenstern (1944) offered a set of axioms for maximization of
expected utility. Their axioms are stated in the language of probabilities, andtheir theory is therefore restricted to situations of risk, rather than uncertainty.
Their theory does not tell us where probabilities might emerge from if they are
not given, and their framework does not support expected utility maximization if
uncertainty is not quantified. Savage’s (1954) main contribution was to extend the1This statement refers to the standard Bayesian decision model. Many economic models augment thismodel with very strong assumptions about beliefs, such as an assumption of rational expectations or anassumption that there is a common prior in multiagent problems. However, the axiomatic foundationsof individual beliefs, which are our focus here, do not imply these additional assumptions. In this paper,
we deal with individual (as opposed to interactive) decision making.

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expected utility paradigm to situations where objective probabilities may not exist.
He showed that axioms on consistency of choice in the face of uncertainty imply
that the decisionmaker behaves as if he had a subjective probability with respect towhich he wishes to maximize expected utility. Deriving probability and utility
simultaneously from observed choices, Savage has provided the most compelling
justification of the Bayesian paradigm: he showed that in order to make coherent
choices, individuals should behave as though they have subjective probabilities,
even when objective probabilities cannot be defined. Savage’s approach merits amore detailed discussion.
Savage’s Axiomatic Approach
Savage’s (1954) axiomatization considers observable choices between pairs of
uncertain acts and rules out patterns of choices that do not seem “sensible.”2 To
understand the axioms, consider bets on a horse race. Suppose you were offered
the following gambles: “If Horse A wins the race, you get a trip to Paris (otherwise
you get nothing)” or “If Horse B wins the race, you get a trip to London (otherwise
you get nothing).” Choices between such gambles reflect both the desirability ofthe outcomes and the probability of the events in question. For instance, if you
chose the first gamble, an outside observer might suspect that you thought that it
was more likely that Horse A would win the race than Horse B, or that you preferred
Paris to London, or that some combination of your beliefs about the likely winnerof the race and your preferences over the two cities led to the observed choice.
Savage suggested axioms, stated in the language of preferences between suchgambles, that suffice for the identification of both a utility function and a probability
measure that jointly characterize the decisionmaker through representation of his
choices by maximization of his subjective expected utility.
Savage postulated four conceptually important axioms.3 The first is the classical assumption, familiar from consumer theory, that preferences are complete andtransitive. Completeness states that for any two bets, the decisionmaker can say whichis (weakly) preferred, that is, which is at least as good as the other. Offered bets “If
Horse A or B wins the race, you get a trip to Paris (otherwise nothing)” and “If
Horse C does not win the race, you get a trip to London (otherwise nothing),” theperson has well-defined preferences between them (including the possibility of
indifference). Transitivity requires that (weak) preference for gamble A over B andfor B over C results in (weak) preference of A over C.
Two additional axioms deal with the separation of tastes from beliefs. Theseaxioms are quite restrictive in particular applications that involve uncertainty about

2

There are other axiomatizations of subjective probability, coupled with the principle of expected utility
maximization: for example, Ramsey (1931), de Finetti (1937), and Anscombe-Aumann (1963). We focus
on Savage’s axiomatization because it is widely perceived to be the most satisfactory from a conceptual
viewpoint.
3
Savage also needed three additional axioms, which can be viewed as technical. They guarantee notionsof continuity and rule out trivial cases. These axioms will not be discussed here.

Probability and Uncertainty in Economic Modeling177

one’s health or survival, where preferences can alter depending on the state inwhich the preferences occur, but these concerns are not the main focus of thispaper, and we therefore do not discuss these axioms here. For a discussion andcritique of these axioms, see Drèze, (1961), Karni, Schmeidler, and Vind (1983),
and Karni (1993, 1996, 1999, 2003).
A fourth axiom, which is crucial for the present discussion, is the Sure Thing
Principle. To illustrate this axiom, consider the following gambles: Gamble G1 is “If
Horse A wins the race, you will get a trip to Paris; and if Horse A does not win you
will get a trip to Philadelphia,” while Gamble G2 is “If Horse A wins the race, youwill get a trip to London; and if Horse A does not win, you will get a trip to
Philadelphia.” The two gambles are identical if Horse A does not win the race, butoffer a choice between Paris and London if Horse A does win. Consider also two
other gambles in which the prizes if Horse A wins are the same, but there is adifferent consolation prize if Horse A doesn’t win. Specifically, let Gamble G3 be “If
Horse A wins the race, you will get a trip to Paris; and if Horse A does not win, youwill get a trip to Montreal”; and Gamble G4 be “If Horse A wins the race, you will
get a trip to London; and if Horse A does not win, you will get a trip to Montreal.”
G1 and G2 differ only in the case that Horse A wins, and then the question iswhether the decisionmaker prefers Paris to London. Similarly, G3 and G4 differ
only in the case that Horse A wins, and again, the question is whether thedecisionmaker prefers Paris to London. The Sure Thing Principle requires that thedecisionmaker prefer G1 to G2 if and only if the decisionmaker prefers G3 to G4.
These and the other Savage (1954) axioms seem eminently reasonable. Savage’s theorem states that if a decisionmaker’s choices are coherent in the sense thatthey satisfy the axioms, then these choices are equivalent to the maximization of
expected utility with respect to a subjective probability measure. That is, a decisionmaker behaves like a person who possesses a probability distribution over thestates of the world (specifying which horse wins the race in the example above) anda utility function over the outcomes (trips to Paris, London, Philadelphia, andMontreal above), and the decisionmaker maximizes the sum of the utilities of theoutcomes weighted by the probabilities that the outcomes will occur.
Why Savage’s Result Matters
Savage’s (1954) theorem that a person behaves as though that person maximizes expected utility is important for several reasons. First, it can be interpreted
normatively: to the extent that the axioms appear reasonable, so does expectedutility theory. Thus, if Ann or Bob were to ask us for a recommended course ofaction in their respective decision problems, we might start by asking them whether
they would like to make decisions consistent with these axioms. Assume that Ann
considers Savage’s axioms and says, “Yes, this is the kind of decisionmaker I’d liketo be” or even, “Now that you explained the axioms to me, I would be embarrassed
to be caught violating these axioms.” We can then quote Savage’s theorem and say,“Well, then, you must behave as if you were maximizing the expectation of a certain
utility with respect to a certain probability measure. It would, perhaps, be easier for

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you to think directly in terms of utility and probability and, once you chose thesefunctions, simply to follow expected utility maximization.”
Second, the axiomatization is useful for descriptive purposes. It delineates thescope of observed phenomena that are consistent with subjective expected utility
theory, in particular, the claim that people maximize expected utility relative to a
subjective probability measure. As such, the axiomatization may also help in testing
subjective expected utility theory. Because direct empirical tests of the theory may
be fraught with identification problems (in real-world situations, isolating effects ofdifferent subjective probabilities or different utilities is likely to be quite difficult),
one may wish to test Savage’s axioms in simple choice situations in the laboratory
or in mind experiments. To the extent that the axioms appear valid in such
experiments, one might be convinced that they are also valid in real choicesituations and, therefore, that subjective expected utility maximization is a goodmodel of the way people make decisions in reality.
To illustrate this point, suppose that economist A, when analyzing the military
career choice that Bob and other young men make, assumes that they makedecisions by maximizing their subjective expected utility. Economist B is skepticalthat this is the right model to use. Neither economist has direct access to thedecision processes of the young men in question. Moreover, the economists do nothave sufficient data on the choices made by these men to test whether theygenerally are expected utility maximizers. If A were to suggest the literal interpretation of expected utility theory, namely, that Bob actually calculates products ofutilities and probabilities, B would find A’s theory bizarre. But suppose that A goesover Savage’s axioms and asks B whether it is plausible that the decisions Bob would
make, given various choice situations, would be in accordance with the axioms andthat B finds these consistency requirements reasonable. Then economist A may
quote Savage’s theorem, convincing B to accept expected utility theory as a descriptive theory of the decisions that will be made to the same degree that B accepts
the axioms as description of behavior. This is not an argument that A will be able
to convince B that maximizing expected utility with respect to a subjective probability distribution is a good description of the process by which the young men reach
their decisions, only that their decisions are the same as though they did so.
Third, Savage’s axiomatization of subjective expected utility maximization canalso facilitate the determination of the subjective probability of various events byfocusing on simple trade-offs. Suppose, for example, that Ann would like to elicit
her subjective probability for the event “Candidate X will graduate successfully fromthe program.” She might ask herself questions: “Do I prefer to bet on X graduating
successfully or on another candidate, Y?” “Am I willing to bet on X graduating vs.
failing at odds 1:2?” Such preference questions may have a more palpable meaning
to Ann than the question, “What is the precise probability p that X will graduate?”
Yet, if Ann satisfied Savage’s axioms, a set of simple binary comparisons like thesewill identify a unique p that can be defined as her subjective probability for the
event in question.

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Difficulties in Savage’s Approach to Rationality
Savage’s axioms are often considered to be the behavioral definition of rationality. If Ann or Bob ask us what will be a rational decision for them to make, weshould point out to them that, if they accept the axioms, then there must be
probabilities and utilities that represent their choices via expected utility maximization. They may then ask us, how they can figure out which probability and which
utility are “theirs”. Here we can resort to Savage’s axiomatization again and tellthem that they should examine their own preferences and elicit their implicit
beliefs. This approach, however, faces several difficulties.
First, Ann and Bob might find themselves expressing preferences that are incontradiction to one or more of the axioms. For example, assume that Ann isconsidering files of two candidates. Candidate X comes from a college Ann knows
well. She has seen many similar students, and she observes that about 60 percent ofthem graduated successfully. Candidate Y comes from a foreign country. Ann has
no experience with students from Y’s college or, in fact, with anyone from Y’s
country. Out of ignorance she might assign to this candidate a success probability
that is the overall success rate for all students in the program. Assume that thisgeneral success rate is also 60 percent. Yet, Ann knows that this number, 60 percent,
was assigned almost as a default. By contrast, the 60 percent probability assigned tothe success of candidate X is based on a significant amount of information. The two
probabilities, though equal, “feel” different. More concretely, we should not be
surprised if Ann is more willing to bet on candidate X’s success than on Y’s.
Typically, Ann might feel safer with a bet whose distribution is known, rather thanwith one whose distribution is not known.
The Bayesian approach, logically necessitated by Savage’s axioms, fails to
distinguish between probabilities based on data and probabilities that result froma default rule (which is another way of saying “ignorance”). Consider bets on two
coins, one which was extensively tested and was found to be fair, and another about
which nothing is known. The outcome of a toss of the first coin will be assigned a
50 –50 distribution due to “hard” evidence. The outcome of a toss of the second
coin will be assigned the same distribution in accord with Laplace’s principle ofindifference. But as Schmeidler (1989) argues, the two distributions feel different,
and, as a result, our willingness to bet on them need not be the same. In a classic
experiment, Ellsberg (1961) has shown that people often express preferences forbets with known probabilities over bets with unknown probabilities.4
Another fundamental difficulty with the descriptive interpretation of the axioms underlying expected utility theory has to do with the completeness axiom,
namely, the assumption that the decisionmaker has preferences between any two

4
There are other reasons why Savage’s (1954) axioms might be violated by observed behavior. Some
involve general critiques of the rational choice paradigm, such as framing effects, gain–loss asymmetry,and other phenomena documented in Tversky and Kahneman (1974). Violations of transitivity wereobserved by Lichtenstein and Slovic (1971). Other problems are specific to the expected utility model,such as state-dependent preferences briefly mentioned above.

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uncertain acts (Shafer, 1986; Bewley, 2002; Gilboa, Postlewaite, and Schmeidler,
2006). Ann and Bob may find that, for many pairs of acts, they simply do not havewell-defined preferences. Specifically, if we were to ask Bob if he preferred to join
the military or not, his reply would likely be: “That is precisely what I am trying tofind out.” Likewise, Ann’s choice between different candidates is the decision
problem for which she is interested in probabilities in the first place.
The completeness axiom is a standard assumption in consumer theory. Indeed, when the outcomes of various choices are certain and known to a consumer,
this axiom is rather innocuous. If Dan is offered a choice between a bowl ofchocolate ice cream and a bowl of vanilla ice cream, Dan is likely to choose aparticular flavor with no hesitation. He doesn’t need to make any calculations
about the options. The consumer has well-defined preferences, which are also
accessible to him through introspection. Correspondingly, if Dan prefers chocolate
to vanilla, no outsider can convince him that his choice is incorrect, and that heactually prefers vanilla to chocolate.
But in the presence of uncertainty, whether about objective outcomes or about
one’s subjective experience, completeness of preferences is a less compellingassumption. Assume that Carol is taking a new job, and she is offered either one
pension plan with defined benefits that depend in a complex way on wages andyears of service, or a plan with defined contributions whose eventual pension
payments will depend on the amount contributed, return on investment, andchoices over types of payouts made at retirement. Carol must make a choice, as
must Dan, who has to choose what flavor of ice cream to have. But Carol’s choice
is very different from Dan’s. Carol is likely to have no a priori preferences between
the plans, unlike the choice between chocolate and vanilla. In Carol’s case, factfinding and reasoning must precede preferences. All that Carol can say a priori isthat, at any point in the future, and at every realization of uncertainty, say, about
her health, she prefers more money to less. But these preferences, which precedereasoning, do not suffice to determine complicated choices between uncertain
prospects. In particular, the two pension plans would have to be analyzed by (or for)
Carol to determine her preferences between them. The analysis of the two plans
calls for the assessment of various risks, that is, for the evaluation of probabilities.
It follows that the evaluation of probabilities is, in many cases, a step in theformation of preferences. Asking Carol what probabilities she assigns to various
events by observing her own preferences is a circular proposition that does notadvance Carol in the evaluation of probabilities or in the formation of preferences.
Observe also that Carol can’t recall her past experiences with the two items onoffer and compare her satisfaction with them, as Dan could do in choosing ice
cream. Carol has not lived through a lifetime of work and retirement before. Even
relying on other people’s experience might not suffice because no one has yet lived
under the economic conditions that will prevail when Carol retires. Indeed, Carol
may go through a complicated mental process, and possibly consult friends andcolleagues, before choosing. Unlike Dan’s ice cream decision, it is quite possiblethat someone could convince Carol that her tendency to prefer, say, the company

Probability and Uncertainty in Economic Modeling181

pension plan was “wrong” by explaining to her the consequences of that choice if,say, the company were to be taken over by another firm. In a simple consumer
problem such as Dan’s, preferences over different kinds of ice cream can be taken
as “primitive.” Primitive preferences need not be immutable: they might change inthe long run as a result of advertisement, new information, or habit formation. Yet
such preferences exist, are available to introspection without the intervention ofreasoning, and for many applications they also appear to be stable in the short run.
By contrast, in Carol’s example, preferences cannot be primitive. Rather, Carol’s
preferences between pension plans are the result of reasoning, or even explicit
calculations that depend on the probability of various events.
We therefore hold that a rational decisionmaker’s preferences may notsatisfy Savage’s axioms. The converse is also true: we may be unwilling to classify
as “rational” every decisionmaker who does satisfy the axioms. The reason is thatthe axiomatic approach places constraints neither on the subjective probabilities, nor on the utility function that can be used to represent preferences. Suchan agnostic position may be reasonable in the case of utility functions; indeed
most introductory textbooks emphasize that consumer rationality is only amatter of internal consistency and does not restrict the consumer’s tastes in any
fundamental way. But a similar agnostic position for the case of probability is far
less defensible. Because probabilistic beliefs can be in accordance with evidenceor at odds with it, some beliefs are more sensible than others. For example,
assume that Ann’s beliefs, as reflected in her choices, assign high probability ofsuccess to candidates graduating from a certain school, despite the fact thatsuch candidates consistently fail in the program. Such beliefs could be considered “unreasonable.” Similarly, many beliefs in supernatural phenomena andmany superstitions are considered “irrational” because they conflict withevidence. Yet, nothing prevents a decisionmaker from holding such beliefs andalso satisfying Savage’s axioms. In other words, Savage’s axiomatic system
restricts choices only to be internally coherent and is therefore insufficient for
an intuitive definition of rationality. In Gilboa, Postlewaite, and Schmeidler(2006), we develop this argument in some detail.
Despite the appeal of the axiomatic justification of subjective expected utility
maximization, the foundation of subjective probabilities remains unsatisfactory foreconomic modeling. A decisionmaker may seek guidance in Savage’s axiomatic
derivation to form probability beliefs, but the axioms are significantly less plausible
than they seem at first glance. Specifically, the decisionmakers involved may findthat they have no a priori preferences over the relevant gambles, or that theirpreferences tend to violate the Sure Thing Principle.
This conclusion leads to two questions: First, are there alternatives tomodeling decisionmakers as maximizing expected utility with respect to given
probability beliefs? Second, if a decisionmaker’s beliefs do not come frompreferences over lotteries, what is the basis for beliefs? We address thesequestions in turn.

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An Alternative Approach: The Multiple Prior Model
Models that assume that decisionmakers maximize expected utility have been
tremendously useful in generating insight into economic behavior under uncertainty. This fact is not contradicted by empirical or experimental violations of
expected utility theory. Indeed, all economic models fail to be perfectly accurate
descriptions of reality. When a useful model is shown to be inaccurate the key
question becomes, is it inaccurate in an important way? Can it lead to qualitatively
wrong conclusions? And, if so, what alternative models might provide better
guidelines for understanding economic situations and generating predictions
about them?
Consider again the example above in which Ann wants to assign a probability
to a candidate from a foreign country being successful when she has no experience
with students from the candidate’s college or from his country. We suggested above
that, out of ignorance, Ann might assign a probability of success equal to thegeneral success rate, say 60 percent. But Ann knows that this number is a default
rate in which she has little confidence. She might feel uneasy about making
decisions that hinge on this particular probability. In fact, she may even feel that it
would not be rational to rely on this assessment, which is somewhat arbitrary. An
alternative that Ann might consider is to model her state of knowledge more
explicitly, and to give up on the notion that she should assign an exact probability
to the event that this student will be successful. Instead, she can assign a range ofprobabilities to this event—say, between 55 and 65 percent.5 Ann might thencalculate her expected utility for each of the probability distributions and make her
decision based on the set of expected values she obtains. For example, if she wishes
to be cautious, she might assign to each alternative the minimum expected value
over her set of priors and choose the alternative that yields the highest suchminimum. Gilboa and Schmeidler (1989) axiomatized the multiple prior model
discussed here. Others models employing multiple priors include Bewley (2002),
Klibanoff, Marinacci, and Mukerji (2005), Seo (2007), and Maccheroni, Marinacci,
and Rustichini (2006a,b). These models are also axiomatically based, that is, they
have characterizations of their respective decision rules by the patterns of behavior
that are compatible with them.
The multiple prior model in which the decisionmaker chooses the alternativethat maximizes the minimum expected utility across the possible priors is clearly analternative to the standard expected utility model. Moreover, ranges of probabilities can reflect the intuitive notion that a decisionmaker might feel more confident
in some beliefs than in others. But does the presumably more intuitive multiple
prior model lead to new insights? The answer is “yes.”5

One could argue that Ann should have a “prior over the set of priors” and simply reduce the problem
to a single prior by computing compound probabilities. However, if Ann does not know what the priorprobability should be, it seems even less likely that she would be confident about a specific prior overthe set of possible priors.

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As one example, the multiple prior model has been applied to the problem of
optimal investment. Suppose that Carol has 100 shares of Intel stock and, in
addition, several thousand dollars in the bank. Carol must decide whether sheshould sell some of her Intel stock, buy more, or leave her portfolio as it is. In thestandard expected utility model (setting aside considerations such as transaction
costs or the possibility that the stock price itself offers information), there will be aunique price at which Carol would neither buy nor sell any stock; at any higher
price she would sell some or all the stock, and at any lower price she would buy
more. This conclusion seems overly sharp. It seems more likely that there would bea range of prices at which Carol would be willing to leave her portfolio unchanged.
Dow and Werlang (1992) showed that in the multiple prior model of Gilboa andSchmeidler (1989), this is exactly what occurs. When Carol’s beliefs are represented
as a set of priors rather than a single prior, she leaves her portfolio unchanged overa certain range of prices.
To see the basic logic of this result, suppose that there are two states of theworld and an asset A that yields $1 in state 1 and negative $1 in state 2. Consider
an investor who holds none of the asset but can buy or sell short a positive
quantity of the asset and who believes that the probability of state 1 is in therange 0.4 ⱕ p ⱕ 0.6. For simplicity, assume that the utility function is u(x) ⫽ x.
If the investor evaluates a purchase of an additional unit of the asset A, thelowest expected utility is obtained at p ⫽ 0.4, and it is – 0.2. On the other hand,
if the investor evaluates a sale of a unit of the asset, the lowest expected utility,
obtained this time at p ⫽ 0.6, is – 0.2 as well. Thus, if the price of the asset is in
the range (– 0.2, 0.2), the investor will prefer neither to buy nor to sell A.
The multiple prior model has also been employed in a job search model by
Nishimura and Ozaki (2004). They ask how an unemployed agent will react toincreasing uncertainty in the labor market. In a Bayesian model, greater uncertainty might be captured by higher variance of the job offers the agent receives.
Other things equal, an increase in variance (holding the mean constant) shouldmake the agent less willing to accept a given offer, knowing that he has a chance toreceive a substantially better offer in the future. However, the counterintuitive
conclusion that greater uncertainty should cause an agent to be more likely to turndown a current offer is the result of the assumption that all uncertainty is quantifiable by a probability measure. Nishimura and Ozaki show that in a multiple priormodel—assuming, again, an “uncertainty averse” agent who uses the “maxmin” ruleof choosing the option whose worst case expected utility is the highest—theconclusion might be reversed: in the presence of greater uncertainty, modeled asa larger set of possible priors, agents will be more willing to take an existing job
offer rather than bet on waiting for better ones in the future.
Hansen and Sargent (2001, 2003, 2007) have applied the multiple prior model
to macroeconomic questions starting from the viewpoint that, whatever probability
model a policymaker might have, it cannot be known with certainty. Considering a
set of priors around a given model and asking how robust economic policy would
be to variations in the underlying probability, they revisit and question classical184

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results. Hansen, Sargent, and Tallarini (1999) compare savings behavior under
expected utility maximization with the savings behavior of a “robust decisionmaker”
who behaves in accordance with the multiple prior model. They show that the
behavior of a robust decisionmaker puts the market price of risk much closer
to empirical estimates than does the behavior of the classical expected utility
maximizer.
In short, the multiple prior model can yield qualitatively different and moreplausible results than a standard Bayesian model. The latter is a good model formany situations where theoretical results remain qualitatively unchanged ifanalyzed with a multiple prior model. However, many theoretical results dohinge on the existence of a prior belief defined by a single probability. When aresult seems to depend on such a prior, one may become suspicious about its
general applicability.

Belief Formation
Savage’s axioms might convince Ann and Bob that they wish to maximize
expected utility relative to a subjective prior, and the axioms may help Ann and Bob
to elicit such a prior if they have complete preferences that satisfy the axioms. But
decisionmakers who attempt to define these preferences via the formation of theirbeliefs will find little help in the axioms. The question remains, therefore, howshould Ann and Bob define prior beliefs? What are reasonable beliefs to hold?
Let us start with Ann. Assume that, trying to predict the success of a new
candidate, she wants to use all the available data about successes and failures of
Ph.D. students in the past. However, she would also like to give more weight in her
probability assessment to data on former applicants who were more similar to thecurrent applicant. One way to do this would be to choose a “similarity function,”
which would measure the degree to which two applicants were similar whenevaluated for possible admission, and then compute the relative frequency of
success among students in the program, weighting each student by his or her
similarity to the new candidate. Thus, the “probability” of success of a candidate
would be the sum of similarities to all students who have succeeded, divided by thesum of the candidate’s similarities to all students who either succeeded or failed.
If the similarity function is constant, the formula reduces to standard empiricalfrequencies, giving equal weight to all past observations, ignoring differences in the
degrees of similarity between these past cases and the one in question. If, however,the similarity function takes the value 1 for cases that are identical to the new one
in all measurable variables, and the value 0 otherwise, the similarity-weighted
formula becomes the empirical frequency in the subdatabase defined by the valuesof these variables. A similarity-weighted relative frequency thus suggests a continuous spectrum between these two extremes. This approach has been axiomatizedin Billot, Gilboa, Samet, and Schmeidler (2005) and in Gilboa, Lieberman, andProbability and Uncertainty in Economic Modeling

185

Schmeidler (2006), building on ideas of case-based decision theory (Gilboa andSchmeidler, 2001).
An obvious question about this approach is that it may appear that the
problem of finding an appropriate probability has simply been replaced by the
problem of finding an appropriate similarity function. However, reducing thequestion of probability to similarity is a meaningful step. In particular, an “objective” similarity function may be computed from existing data, by finding thefunction that best fits the data, if we were to use the similarity-weighted frequency
as our prediction formula. We can illustrate this process with Ann’s problem.
Ann would start by choosing a certain parameterized family of similarity
functions. For each function in this family, Ann could imagine going over her
database of Ph.D. students, and asking for each observation i, what her prediction
would have been if she were asked to predict the outcome of case i, given all othercases in the database, and using the specific similarity function at hand. She would
then select the similarity function that minimizes the sum of squared errors of thesehypothetical predictions relative to the observed realizations. The similarity function that minimizes the sum of squared errors would then be used to assess
probability of success in the next observation. Gilboa, Lieberman, and Schmeidler(2006) refer to the resulting function as the “empirical similarity.”This approach is offered as a model of the way in which the decisionmakerforms beliefs. There are several benefits to such an approach. First, this approachemphasizes that beliefs do not typically arise from introspection. Rather, beliefs
often result from conscious assessments by the decisionmaker. Those assessments
take as raw data past experiences, observations, and conversations with others. They
may not always follow a precise calculation as offered by the model, but they do
involve the weighing of different observations and the learning of the weights
themselves from experience as well. Opening what is often taken to be a black box
and analyzing the process of belief formation allows us to make inferences about
which beliefs are more reasonable than others for a decisionmaker. In particular,
many economic problems have to do with equilibrium selection, as in the case ofpredicting market bubbles and crashes. The problem of equilibrium selection isfundamentally a problem of belief formation: the equilibrium that will be played isthe equilibrium that is believed to be played. If we can predict which beliefs will be
generated, we can also predict which equilibrium will result.
Second, all other things equal, models that are more descriptively accurate are
preferable. The primary test of a model is the insight from the predictions of themodel. But the way in which a problem is modeled also affects the subsequent
questions that a researcher might ask. If instead of taking beliefs as exogenous,
researchers also model the formation of those beliefs, researchers will also be led
to question the relative importance of different factors on beliefs. Shea (2003) asks
whether the beliefs of children whose parents were financially devastated by theGreat Depression systematically differ from those whose parents were spared. Do
people who have been laid off put greater probability on there being a serious

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recession in the next five years? These are examples of questions of fundamental
importance that require modeling the formation of beliefs.
Modeling the belief formation process may also help in understanding whena particular model of beliefs is applicable. Specifically, consider Bob’s problem. If
we were to apply the similarity-weighted frequencies approach to this problem, theapproach may seem less satisfactory than in Ann’s problem. For instance, the factthat the United States was involved in two Gulf Wars in the past two decades neednot make it more plausible that the United States will be involved in another such
war in the coming decade. In fact, the causal relationships between consecutive
wars are intricate enough to support a variety of conclusions about the possibility of
another such war. In the first problem, Ann had data about many students, whose
successes and failures could be assumed causally independent from each other.
Even though the students were not identical, certain notions of probability could be
defined, be it by the empirical similarity-weighted frequencies or by logistic regression. By contrast, Bob is facing a problem with relatively few cases, which areanything but causally independent. There may be no rational, reasoned way toassign probabilities in Bob’s problem, and one may have to do with less-structured
models (such as the multiple prior model mentioned above). The exercise ofmodeling belief formation does not help us only in finding reasonable beliefs to be
used in our models; it can also help us delineate the scope of applicability ofprobabilistic models.
The Bayesian approach does not suggest a model of the formation of priorbeliefs, but it describes the way that these beliefs are updated—namely, according
to Bayes’s rule. While our main point is that economists should delve into theprior-formation process, we also note that Bayesian updating is unlikely to be theonly way in which beliefs are updated. In particular, Bayesian updating cannot
account for adjustments of beliefs in the absence of new information. There aremany instances in which people adjust their beliefs in the face of arguments that do
not present new information but suggest that different conclusions should be
drawn from the same body of knowledge. Aragonès, Gilboa, Postlewaite, andSchmeidler (2005) argue that, due to computational complexity considerations,such “fact-free” learning is unavoidable even if agents are rational. This argument
suggests that economists should be interested in realistic models of the way beliefs
are formed, as well as of the way they are updated.

Conclusion
Models relying on the standard expected utility approach, which identifies a set ofoutcomes and places a probability on each outcome, have led to fundamental insights
in virtually all fields of economics. At the same time, we find this approach lacking in
two important ways, both of which may restrict the insights gained from formal models.
The first limitation is that the standard expected utility model, along with
Bayesian extensions of that model, restricts attention to beliefs modeled by a single

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187

probability measure, even in cases where no rational way exists to derive suchwell-defined beliefs. We have argued that allowing more general models of beliefs
may lead to more realistic results. The second limitation of the Bayesian frameworkis its silence regarding the origins of beliefs. We argued that a better understandingof the process by which beliefs are generated, whether these beliefs are Bayesian ornot, may help researchers in providing better predictions, as in the case of equilibrium selection problems.
Both limitations of the standard expected utility approach and the Bayesian
approach are partially dealt with in classical statistics. The classical statistical inference
problem is defined by a set of probabilities, or distributions. Point and interval estimation consist in choosing a distribution or a set of distributions (respectively) from theoriginal set. Hypothesis testing deals with the possibility of trimming the original set toa subset thereof. Relatedly, statistics explicitly models the formation of beliefs based on
data. Empirical work in economics typically employs classical (non-Bayesian) statisticaltechniques, such as hypothesis testing and interval estimation. As such, it implicitly
adopts an approach in which a set of probability models is considered possible, andobservations are used to trim down that set. Economic theory may benefit fromallowing economic agents to be non-Bayesian as well and from explicitly modeling theway people form beliefs based on observations.

y Our thinking on these issues was greatly influenced by discussions with many people. We areparticularly grateful to the editors who have provided help and advice throughout the preparationof this paper. Gilboa and Schmeidler gratefully acknowledge financial support from Israel Science
Foundation grants, Nos. 975/03 and 355/06, the Pinhas Sapir Center for Development, and thePolarization and Conflict Project CIT-2-CT-2004-506084, funded by the European Commission-DG Research Sixth Framework Programme. Postlewaite gratefully acknowledges support fromthe National Science Foundation grants Nos. SES-05-27656 and SES-03-50969-002.

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