From: tai@ailab.ncsu.edu (Scott Tai)
Newsgroups: rec.org.mensa
Subject: (T) INTELLIGENCE DEBATE: PART I
Date: 13 Apr 92 04:01:01 GMT
Reply-To: tai@ailab.ncsu.edu (Scott Tai)
Organization: NCSU Artificial Intelligence Systems Laboratory

-------------------------
INTELLIGENCE AND HEREDITY
-------------------------

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.  (Most of them are now in their fifties.)  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 Eurpoean 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.
	
Identical Twins Reared Apart
----------------------------

	One of the most important lines of evidence for the inheritance of
intelligence in humans comes from studies of identical twins who were separated
shortly after birth and reared in different homes.  Identical twins
originate from
a single fertilized ovum which splits in the course of early development
to form
two individuals.  Each member of the pair of twins therefore has exactly
the same
complement of genes.  Consequently, any difference between the twins
must be due
entirely to non-genetic or environmental differences.
	Twins separated shortly after birth are often reared in families that
differ markedly in social class, and the range of environmental
differences
observed in their foster homes is fairly typical of the environmental
variations
seen in the general population.
	Four major studies of identical twins reared apart, conducted in
England,
Denmark, and the United States, and totaling 122 pairs of twins, are in
remarkably
close agreement in showing that twins reared in different homes are
still much
more alike in I.Q. than are fraternal twins reared together.  Fraternal
twins are
merely siblings who happen to be conceived and born at the same time,
and therefore
half of them are of the opposite sex.  In I.Q. and other traits they
resemble one
another no more than do ordinary siblings born at different times.
	Identical twins reared apart differ, on the average, by only 6 to 7
points.
But even if we test the very same person on two occasions a week apart,
we find
that his test score will vary, on the average, by 2 or 3 I.Q. points. 
This is the
test's "measurement error."  When we eliminate this error from the twin
data, we
find that the twins differ only 4 or 5 points in I.Q.  Identical twins
reared
together differ by only 2 or 3 points, not including measurement error. 
The
largest I.Q. difference ever found in a pair of identical twins reared
apart is 24
points.  More than 17 percent of siblings reared together differ by more
than 24
I.Q. points.  The same is true of fraternal twins.  But siblings (and
fraternal
twins) have only half of their genes in common, and they differ on the
average by
12 I.Q. points (excluding measurement error), even when reared
together.
	The studies of identical twins show clearly that individuals who are 
genetically identical are almost as much alike in mental ability as they
are alike
in physical traits, and this is true even when they have grown up in
different
environments.

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 Effect of Inbreeding on 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
263 / 2719 / 17209
Newsgroups: rec.org.mensa