From: scott@ailab.ncsu.edu (Scott A Tai)
Subject: Heritability of Intelligence
Reply-To: tai@m-net.ann-arbor.mi.us
Organization: NCSU Artificial Intelligence Systems Laboratory
Date: Thu, 30 Jul 1992 03:06:08 GMT


ARTHUR R. JENSEN 
THE HERITABILITY OF INTELLIGENCE
--------------------------------

	The question of why people differ in intelligence has been
asked for centuries, but a scientifically acceptable answer did not
become wholly possible until psychologists devised techniques for
measuring intelligence quantitatively and objectively.  The first
really useful intelligence test was devised in 1905 by the French
psychologist Alfred Binet.  Binet's early test was later revised and
improved by Lewis Terman at Stanford University; the now-famous test
that resulted from these efforts is known as the Stanford-Binet
Intelligence Scale.  It is still the most widely used test of general
intelligence.
	There are other intelligence tests, and although many of them
appear to be quite different from one another, all actually measure
much the same general ability.  That is to say, if we administer
several seemingly quite different intelligence tests to a large number
of persons, their scores on the tests will be pretty much in the same
rank order.  Those who score high on one test will tend to score high
on the others, and those who score low on one test will usually score
low on all the others.  This fact of correlation among all tests of
intelligence led Charles Spearman, the famous English psychologist, to
conclude that there is a general factor, "g," which is common to all
tests of intelligence.  We know that it is practically impossible to
make up a mental test having any degree of complexity which does not
involve "g." We can perhaps most clearly characterize "g" as an ability
for abstract reasoning and problem solving, for seeing relationships,
and for grasping concepts.
	A person's score on an intelligence test is usually expressed
as an I.Q. (for Intelligence Quotient).  The test is standardized in
the general population in such a way that the average I.Q. at any age
is set at 100, and the middle 50 percent of this population falls
within the so-called average range of I.Q.'s going from 90 to 110.

Significance of the I.Q.  
------------------------

	Can the I.Q. tell us anything of practical importance?  Is it
related to our commonsense notions about mental ability as we
ordinarily think of it in connection with educational and occupational
performance?  Yes, indeed, and there is no doubt about it.  The massive
evidence from psychological, educational and industrial research, and
research in the armed forces, is unequivocal.  We know, for example,
that no other single fact that we are now able to ascertain about a
child gives us a better prediction of his future scholastic performance
than his I.Q. obtained after age 5 or 6.  (Below this age I.Q. tests
become less accurate indicators of the child's later mental
development, and below 2 or 3 years of age test scores have practically
no predictive value.)
	The I.Q. obtained after 9 or 10 years of age also predicts
final adult occupational status to almost as high a degree as it
predicts scholastic performance.  When various occupations are ranked
for average income and for the general public's average judgment of the
occupation's prestige and desirability, this rank order is found to be
highly related to the average I.Q. level of the persons in these
occupations.  There is of course a wide spread of I.Q.'s in nearly
every occupation, but the average I.Q. of persons within a particular
occupation is closely related to that occupation's standing in terms of
its average income and the amount of prestige accorded to it by the
general public.
	One of the most convincing demonstrations that I.Q. is related
to "real life" indicators of ability was provided in a classic study by
Terman and his associates at Stanford University.  In the 1920's they
selected a total of 1,528 children with Stanford-Binet I.Q.'s above
140.  The average I.Q. of the group was 152.  These children were
investigated periodically over the years up into their adulthood.
Terman found that for the most part these high-I.Q. children in later
adulthood markedly excelled the general population on every indicator
of achievement that was examined:  a higher level of education
completed; more scholastic honors and awards; higher occupational
status; higher income; production of more articles, books, patents, and
other signs of creativity; more entries in Who's Who; a lower mortality
rate; better physical and mental health; and a lower divorce rate.
Also, they have much brighter children that the average; their average
I.Q. is 133, a level which is exceeded by only 2 percent of all the
children in the general population.
	Findings such as these establish beyond a doubt that I.Q. tests
measure characteristics that are obviously of considerable importance
in our present technological society.  To say that the kind of ability
measured by intelligence tests is irrelevant or unimportant would be
tantamount to repudiating civilization as we know it.

The Causes of I.Q. Differences 
------------------------------

	The layman usually asks:  "Is intelligence due to heredity or
environment?" The scientist promptly answers:  "Both."  Without
heredity and environment there simply is no intelligence.  Obviously
every person must have had a biological inheritance of genes from his
parents and must have grown in an environment, or he wouldn't even be
here to take an I.Q. test.  So, of course, both heredity and
environment are essential for the existence of the individual or any of
his physical and mental characteristics.
	But when scientists actually study this problem, we find that
they do not even ask the layman's question.  The question to which
scientists have sought an answer can be stated as follows:  How much of
the variation among persons in a given population is attributable to
differences in their environments and how much to differences in their
genetic endowments?
	Numerous studies conducted by psychologists and geneticists
over the last 40 or 50 years provide an answer to this question.  The
answer is unambiguous and is generally agreed upon by all scientists
who have considered all the evidence.  This evidence strongly supports
the conclusion that genetic factors are much more important than
environmental influences in accounting for individual differences in
I.Q.  How much more important?  The evidence indicates that genetic
factors account for at least twice as much of the variation in I.Q.'s
as environmental factors.  This conclusion has one main limitation.
Since all of the major studies in this field were conducted with
samples of Caucasian European and North American populations, we cannot
confidently generalize their conclusions to other populations,
especially those with very dissimilar environments.
	What are the kinds of evidence that lead to the conclusion that
genetic differences outweigh environmental differences in accounting
for individual differences in I.Q.?  Most of this evidence, as it is
found in the scientific literature, depends upon quite technical
methods of analysis developed in a specialty known as quantitative
genetics or population genetics.  Some of these methods were devised
originally to analyze the roles of heredity and environment in
agriculture and animal breeding.

Experiments in Animal Breeding 
------------------------------

	Experiments in which we explicitly try to breed for some
specific trait give us the most certain evidence that variation in the
trait has a genetic component.  Psychologists have bred rats for speed
of learning mazes, which is a good indicator of rat intelligence.  By
always mating the fast-learning males with fast-learning females, and
mating slow-learning males with slow-learning females, it is possible,
within 6 to 10 generations, to produce two quite distinct strains of
rats in respect to maze-learning ability.  The slowest- learning rat of
the "bright" strain will learn mazes faster than the fastest rat of the
"dull" strain.  The two strains will differ markedly in the number of
tries they need to learn how to run through a maze efficiently,
avoiding the blind alleys.  These experiments definitely prove that not
only physical characteristics but some behavioral traits as well are
largely inherited through the parental genes.  Thus we should not be
surprised to find in humans that differences in some behavioral
characteristics, including intelligence, are a product of genetic
inheritance.

Unrelated Children Reared Together 
----------------------------------

	The opposite situation to identical twins reared apart is that
of genetically unrelated children adopted at birth by foster parents
and reared together.  Such children differ from one another, on the
average, by 15 to 16 I.Q.  points (excluding measurement error).
Compare this with the 17 to 18 I.Q. points difference between unrelated
children reared in different homes, or the 15 to 16 points difference
between unrelated children brought up in different homes but in the
same socioeconomic class.  We see that unrelated children brought up
together in the same home differ from one another in I.Q. at least 3 or
4 times more than genetically identical twins reared in different
homes.  And the unrelated children reared together differ almost as
much in I.Q. as unrelated children simply picked at random from
different homes.
	The I.Q.'s of adopted children also show little or no
relationship to the I.Q.'s of their adopting parents, but they are
almost as closely related to the I.Q.'s of their natural parents as we
find in the case of children who are reared by their natural parents.
	Children reared in the common environment of an orphanage
differ from one another in I.Q. to approximately the same degree as
children picked at random from the total population.  The I.Q.'s of
orphanage children who have never known their own parents show almost
the same degree of correlation with their parents' level of ability as
we find in the case of children reared by their own parents.

Resemblance Between Parents and Children
----------------------------------------

	Now and then we notice that very bright parents can have an
intellectually mediocre child, or that rather dull parents can have an
exceptionally bright child.  These observations are often pointed to
mistakenly as evidence that intelligence is not inherited.  But the
fact is that genetic theory predicts precisely that we should find such
discrepancies between parents and their offspring.  For example,
parent-offspring differences in height are of about the same relative
magnitude as their differences in I.Q.  Children resemble their parents
physically and in mental ability to about the same degree that they
resemble their own siblings.  The average I.Q. difference between a
parent and his (or her) child is the same as the difference between
siblings--that is, about 12 I.Q. points.  The difference between a
child and the average of both of his parents' I.Q.'s is about 10
points.
	A parent with a high I.Q. will usually, but by no means always,
have children whose I.Q.'s are somewhat lower than his own but are
still above the average for the general population.  A parent with a
low I.Q., on the other hand, will usually, but not always, have
children whose I.Q.'s are somewhat higher than his own but are still
below the average of the population.  This phenomenon, discovered by
Sir Francis Gaulton, is called "regression toward the mean," and it
holds true for height and other inherited physical traits as well as
for I.Q.

I.Q.'s of Husbands and Wives 
----------------------------

	It is interesting that in our society husbands and wives are at
least as much alike in I.Q. as brothers and sisters.  If men and women
picked their mates strictly at random, as by a lottery, spouses would
differ by an average of 18 I.Q.  points.  But in fact men and women
choose one another partly for intelligence, and so spouses differ by
only 10 or 11 points in I.Q.

The Relative Effects of Heredity and Environment
------------------------------------------------

	How can we summarize briefly what is now known about the
relative importance of heredity and environment in causing individual
differences in I.Q.?  In the terminology of genetics a summary answer
consists of saying that the "heritability" of I.Q. is close to 0.80.
This means that 80 percent of the "variance" in I.Q.'s in the general
population is attributable to genetic differences and 20 percent is
attributable to non-genetic or environmental differences.
	"Variance" is essentially a quantitative index of the total
amount of differences that exist among all members of some population.
So instead of talking about variance, we can more easily describe our
conclusions in terms of average differences.
	If we should determine the differences in I.Q. between every
person in the population and every other person, the average of all
these differences would turn out to be 18 I.Q. points.  These
differences are due both to genetic and environmental factors.  Now we
can ask theoretically:  What would be the average I.Q. difference among
all persons in the population if everyone had grown up in identical
environments from the moment of conception, while genetic differences
remained as they are?  Under this hypothetical condition of completely
equal environments for everyone, the average I.Q. difference would be
16 points.  Thus, there would be a reduction of 2 points in the average
difference that now exists.  Let us now ask the reverse:  What would be
the average difference if everyone had exactly the same genetic
endowment, but environmental differences remained unchanged?  Under
this hypothetical condition of complete genetic equality the average
I.Q. difference among persons would be only 8 points, or just half the
difference that would exist with equal environments.
	So the conclusion we come to--which is certainly valid at least
in the white European and North American populations in which the
research was conducted-- is this:  In accounting for the causes of the
differences among persons in I.Q., the genes outweigh the effects of
environment by 2 to 1.  As environmental conditions are improved and
made more alike for all persons in the society, the average
intelligence level of the population will be somewhat increased, and
the I.Q. differences among persons will be slightly reducted.  But of
course the differences that remain will inevitably be due even more to
genetic factors.


-- Scott Tai
   tai@m-net.ann-arbor.mi.us
   255 / 2283 / 14557

From: Craig.Wagner@p2.f120.n109.z1.fidonet.org (Craig Wagner)
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