IQ: Nature Versus Nurture
The
following article appeared in the Wall Street Journal and was carried by the
Associated Press. It appeared on the
SFGate.com web page and may still be there in the archives section at the URL
given below.
Good
genes count, but not only factor in high IQ
SHARON BEGLEY, The Wall Street Journal
Friday, June 20, 2003
URL:
http://www.sfgate.com/cgi-bin/article.cgi?file=/news/archive/2003/06/20/financial0857EDT0029.DTL
Begley: For a trait so highly heritable,
intelligence has been awfully reluctant to give up its genes.
There is wide agreement that cognitive
ability at least partly reflects the influence of DNA: Dozens of studies of
thousands of twins have shown identical twins, who share the same genes, tend
to have more-similar IQs than do other sibling pairs, and children match the IQ
of their biological more than their adoptive parents.
Williams:
The human genome has only recently been mapped. At this point, we know possibly as much about what each gene does
as Newton knew about the components of the universe.
The
comments concerning twins and adoption are correct, but considerably
understated. Twin studies have revealed
a great deal more than just greater similarity than is found in other sibling
pairs. For example, the IQs of
identical twins reared apart (MZA) are more similar than fraternal twins reared
together (DZT).[1]
With
respect to adoption, Jensen writes: “The IQs of adopted persons who have never
known their biological parents are more highly correlated with the IQs of their
biological parents than with the IQs of their adoptive parents. Unrelated persons who were reared together
from infancy show a much lower IQ correlation with each other in early childhood
than do biological siblings, and they show virtually zero IQ correlation in
adolescence and adulthood.”[2]
Begley: Together, these
studies imply genes account for about 50 percent of the difference in
intelligence from one person to the next. That's a high enough
"heritability" that you'd think genome labs would be practically
spitting out genes related to intelligence.
Williams: Present day estimates of the heritability of
intelligence are higher than 50%.
Bouchard[3] estimated
that the variance (h2) is 70% for adults.
"By
adulthood, all of the IQ correlation between biologically related persons is
genetic. In other words, to the extent
that there is a correlation between the IQs of genetically related
post-pubertal family members, the correlation is entirely due to genetic
factors; the environmental contribution to the familial correlations is
nil."[4]
Begley: Intelligence has many meanings,
but what scientists call general cognitive ability seems to reflect memory
skills, verbal and spatial abilities, and abstract reasoning. Usually, if
you're good at one, you're good at the others. Although that correlation may
reflect not "brain quality" alone but something nonphysiological,
such as differences in motivation, it has inspired a search for genes that make
better brains.
Williams:
There is no doubt but that motivation and other non-intelligence factors are
very important to life experiences and achievement. If the subject is intelligence, it should be focused on
psychometric g and the small contribution from the non g
components of group factors.
Begley: … Similarly, purported IQ genes
may cluster, by chance, in groups whose culture values education, yet not
actually make a brain smarter.
Williams:
I disagree. Intelligence is not the
product of education. Intelligence is
the ability which is reflected in psychometric g. It is most likely the product of physiology,
which is genetically determined. That a
culture may value education or not does not change the genes and it does not
change psychometric g. If
education (or the act of valuing education) were a cause of intelligence, we
would have longitudinal studies in which those individuals who pursued much
education would have ever increasing IQs and those who dropped out of school
would not show IQ gains. This is not
what such studies have shown. The
reality is that intelligence differences found at about the age people enter
school are the same as found later in life.[5] IQ stabilizes long before most people
complete their formal educations. The
extent to which IQ measurement is influenced by education is most likely do to
the reduction in g loading which occurs when the novelty of a test is
removed. In such cases, there is a
corresponding increase in s (specificity).
Related
to this is the phenomenon of child prodigies.
We have well documented cases of young children demonstrating
extraordinary abilities as children.
True prodigies[6]
exist in music composition, chess, and mathematics, but we have cases of
extraordinary intelligence which has permitted children to complete college
many years before their age peers. This
cannot be explained by cultural or familial values.
"IQ
at age five-and-a-half accounts for 50% of the variance in Mathematics scores
at sixteen-and-a-half years of age. This is a sobering finding for those
educational advisers who have been abandoning the use of tests of general
intelligence over the past few years."[7]
Begley: There's another problem.
Neuroscientists can't find any fundamental brain processes that distinguish
Einstein from the rest of us -- not speed of neuronal transmission, not the
ability to form synapses, not the quantity and quality of neurons, Prof. Plomin
says. That makes it less likely that genes for those basic characteristics (even
if scientists find them) have a significant effect on intelligence.
Williams:
Diamond et al. (1985)[8]
reported that one area of Albert Einstein’s brain had an exceptionally high
ratio of glial cells to neurons. Since glial cells create and maintain myelin
(Vom Muralt, 1972, p. 5; Kandel, 1991, p. 22), this would be consistent with
thicker myelin contributing to greater intelligence. Also, suggesting that a
real effect exists, Diamond (1988) found that rats reared in an enriched
environment had an elevated ratio of glial cells to neurons.[9] His brain is also reported to have had a
larger than normal parietal lobe.[10]
Begley: Even if the newly suspect
intelligence genes hold up, they will surely turn out to be only the tip of a
huge iceberg. It looks more and more as if intelligence reflects the complex
interaction of scores of genes with each other and the environment. No one gene
makes more than a tiny difference. Different forms of CHRM2, for instance,
account for a spread of only three or four IQ points, while CTSD may account
for perhaps 3 percent of the variation between people.
Williams:
As of the present, there is no way to know how many genes influence
intelligence, nor how potent is the effect of any one gene. I believe it is reasonable to expect that
some genes will ultimately be shown to be very important with respect to
intelligence, while others have second or third order influence. If we were to accept the notion that there
are very many genes in play and that they each have only tiny influence, I
think we would have to expect that the range of intelligence would be much
smaller than is actually found and that the heritability of intelligence would
be slight. The reason for both is that
the probability of getting many variables to go in the same direction must
decrease as the number of variables increases.
As
an example of a large (1500) group which has expertly collected IQ data,
consider the well known Terman study[11]
of individuals with IQ at 140 or above:
subject average IQ = 152, spouse average = 125. The reported
distribution for the children was a normal curve, with a mean of 132.7.
Begley: The heritability of intelligence
may, paradoxically, reflect the importance of environment. If Susie is born
with a slightly better brain than Mary, she will like school, receive more
praise from her teachers, haunt the library, take more demanding courses. In
short, she will bootstrap her way to greater intelligence.
Williams:
Perhaps. There is a concept, referred
to as emergenesis by David Lykken.[12] It basically says that people find their
genetic abilities and arrange their lives in order to hone their natural
abilities. To some extent, this is only
common sense. The problem arises when
people extrapolate that intelligence is caused by self-selecting behavior, such
as study. When a person studies something, he learns the material and may learn
it to the point that he can automatize[13]
some information processing tasks. This
is an ability, not intelligence.
The
notion that macro environmental conditions can affect intelligence is largely
limited to injury or extraordinary circumstances. Sandra Scarr: "Within the range of 'humane
environments,' variations in family socioeconomic characteristics and in
child-rearing practices have little or no effect on IQ measured in
adolescence." Jensen: "There
is simply no good evidence that social environmental factors have a large
effect on IQ, particularly in adolescence and beyond, except in cases of
extreme environmental deprivation."[14]
There
is an additional problem, when one argues that macro environmental conditions
influence intelligence. It is that
there are a number of physical correlates to intelligence which are not likely
to be altered by such human practices as study, thinking, praise, etc. We know that smart brains are physically
larger; they operate with lower glucose uptake; they have higher nerve
conduction velocities; they respond more quickly to such chronometric measures
as inspection time; they respond with less skew to chronometric measurements;
they have more complex electroencephalography traces (from average evoked
potential measurements); and they display a variety of other chemical and
physical properties which differ from less intelligent brains. While these attributes are statistical and
are not individually deterministic, the weight of such physical evidence
strongly supports the notion that what is being observed as a variation in
intelligence is to a large extent physiological. Both chronometric and electroencephalography
measurements can be done in such a way as to measure intelligence with a
correlation to IQ tests which is as good as the correlations from one standard
IQ test to another.
Begley: That explains why the measure of
heritability of intelligence rises with age, from 40 percent in childhood to 60
percent in adulthood.
Williams:
There may be room to quibble about those numbers. I have already presented sources which give the adult value as
70%.
Begley: It isn't that genes grow
stronger. Instead, says James R. Flynn of the University of Otago, Dunedin, New
Zealand, a slight genetic edge at birth snowballs by nudging people to choose
intelligence-enhancing experiences. The result is "a potent
multiplier," he says in Current Directions in Psychological Science.
Williams:
It is interesting that the reference above is James Flynn. My observation is that his beliefs about how
environmental factors influence intelligence are not shared by most
psychometricians. It would make much
more sense to put his ideas into perspective by noting that the vast majority
of psychometric researchers do not suggest the things found in Flynn’s
papers. When Flynn first began
discussing the secular increase in intelligence, he scoffed that the rise could
not be real and gave examples to demonstrate his belief. [One example of this is mentioned later (see
the 100 years ago comment).] However,
he recently wrote a paper (coauthored by William Dickens), titled “Heritability
Estimates Versus Large Environmental Effects: The IQ Paradox Resolved,” in
which he argued that the secular rise was real (this strikes me as a complete
reversal of his prior comments) and that it could be explained by assuming
mathematical relationships between environmental conditions and IQ. The paper is entirely speculative. It does not stand on scientific observation
and is a perfect example of an attempt to prove a preconceived outcome by
fabricating a justification out of thin air.
If
we look at the words of the most credible psychometrician alive,[15]
he says: “The heritability of IQ, in fact, increases from early childhood to
later maturity; that is, the genotypic variance becomes increasingly expressed
in the phenotypic variance throughout most of the human life span. (A predominantly environmental theory of IQ
variation would have to predict the opposite.)”[16] Flynn’s argument is largely environmental
and is not supported by the increase in heritability with age (this, despite
spin to create an opposite impression).
Begley: Prof. Flynn discovered that IQ
soared in recent decades.
Williams:
The term “Flynn Effect” is used today because James Flynn focused a lot of
attention to the secular rise in IQ.
The effect was first identified in the 1930s (not by James Flynn).[17]
Begley: Since 1950, scores on one IQ
subtest have risen 18 points per generation in the Netherlands, Belgium, Israel
and Argentina; between 1948 and 1989, Americans gained the equivalent of 20 IQ
points. The genes we have don't change fast enough to explain this "Flynn
effect" -- but which genes are turned on might.
Williams:
It may be a bit premature to jump to the conclusion that anything other than IQ
scores are rising. There was some
evidence from WISC and WISC-R testing in France and Japan which suggested that
there are verifiable g loaded gains.[18] That information, however, is now in
conflict with other findings.[19]
Among
the curiosities of the Flynn Effect:
“When the g loaded test is composed largely of nonscholastic
items (matrices, figure analogies), the raw scores show a secular increase;
when an equally g loaded test is composed of scholastic items (reading
comprehension, math) the raw scores show a secular decrease. Obviously, the sure level of g cannot
be changing in opposite directions at the same time. The difference in vehicles must account for the discrepancy. So, the extent to which the level of g
per se has been rising (or falling) over the past few decades remains
problematic.”[20] (Rushton and Jensen now suggest that the g
loading is nil.)
It
also happens that people often forget to state the full range of what is known
about the Flynn Effect. For example,
that the effect is not symmetrical with respect to the assumed IQ distribution. The number of low IQ measurements has
decreased more than the number of high scores has increased, so the average
moves upward, but the cause of the shift is not symmetric.
The
magnitude of the Flynn Effect suggests things that most of us would agree is
not a reflection of reality. "Flynn
concluded that someone scoring in the 90th percentile 100 years ago would be in
the fifth percentile today."[21] What does it mean? One of the most likely conclusions is that there is some artifact
of testing that accounts for all or part of the Flynn Effect. How else could we find both increased and
decreased IQs measured over the same time frame?
If
Flynn had held to this position, he would be correct today. We now know that the secular gains are not g
loaded: “The Flynn effect for Wechsler
tests shows it to be unrelated to the g factor; that is, the magnitudes
of the secular gains on the various subtests are not at all related to the
subtests' g loadings (Rushton, 1999).[22]
Apparently the secular gains in Wechsler IQ are attributable to gains in the
non- g variance components in the subtests rather than to their common
factor, g. What these non- g components consist of in terms of
group factors or specificity is as yet undetermined.”[23]
So, if Rushton and Jensen are right, the Flynn Effect does not reflect gains in
g, which resolves the issues previously raised by Flynn concerning the
relative intelligence of people over the past 50 to 70 years.
Begley:
Perhaps growing up with enough leisure time to play chess and even
videogames, or living in smaller and more affluent families that can indulge
children's intellectual curiosity, turns up the activity of genes related to
intelligence. For that reason, says Prof. Plomin, the holy grail in this field
is identifying what experiences turn on genes that influence intelligence.
Williams:
People have been trying for years to find ways to boost IQ through child
rearing practices, mental activities, etc.
Some of the programs designed to increase IQ (such as Head Start)
failed, but managed to achieve self-perpetuation by political means. Even the most intense possible form of
intervention has failed to alter IQ beyond childhood. Children grow up to resemble the IQs of their adopted parents by
the same degree that they resemble the IQs of randomly selected individuals. By age 17, any gains seen by a child who was
adopted into a higher IQ family are gone, but those who were adopted into lower
IQ families statistically rise to higher IQs than those of their adoptive
parents.
We
know that there are indeed environmental components to IQ. They are, however, not traceable to home or
institutional nurture, but rather to circumstances which relate to
biology. Jensen: “… my analysis indicates
that the IQ 'losses' due to the random microenvironment somewhat exceed the 'winnings.'
That is, the microenvironmental factors unfavorable to mental development
appear to be either more potent or more prevalent than those that are
favorable.
Much
of the random microenvironment effect on IQ is probably biological and
originates from the moment of conception. It is related, for example, to
maternal age, parity, immunological incompatibilities between mother and child,
variation in birth weight, obstetrical practices, and other perinatal and
postnatal effects, including traumas and childhood diseases add to the
microenvironment. Then there are other purely random non-genetic effects that
might be called 'para-genetic,' as they are carried by genes that are 'tagged'
or 'imprinted' by certain molecules; but the essential genes, though 'imprinted,'
are not themselves a part of the individual's genotype for a given trait. Yet
these 'tagged' genes cause differences in fetal size and birth weight
independent of mother's age, size, or parity; low birth weight especially
affects IQ unfavorably.”[24]
Begley: Even before that happens, it's
already clear that, with so many genes involved in IQ, genetic engineering for
it isn't in the cards.
Williams:
I see no evidence to support the notion that a single gene could not cause a
huge boost in intelligence. Which
gene? The one which determines the
degree of myelination in the brain is a good candidate. Miller’s model of brain function,[25]
based on neural noise, is probably the best available. Neural noise is suppressed by myelin,
thereby greatly enhancing cognitive tasks.
But myelin presumably contributes to intelligence via an additional
mechanism. Nerve conduction velocity
(NCV) is faster when there is more myelination. High NCV correlates positively with intelligence. Presumably the mechanism for this lies in a
more efficient utilization of the highly volatile working memory.
Begley: If we care about intelligence,
we must seek ways to nurture it not in the genes we pass on to our kids, but in
the world we make for them.
Williams:
If we care about intelligence, we should encourage its nurturing into practical
abilities and discourage dysgenic practices.
So far, we do not have any macro environmental means of enhancing
intelligence and our best present understanding points in other directions.[26]
[1] Jensen, A. R., The g Factor, 1998. P. 177.
[2] Jensen, 1998, P. 177.
[3] Sources of human psychological differences: the Minnesota study of twins reared apart. Thomas J. Bouchard Jr.; David T. Lykken; Matthew McGue; Nancy L. Segal; Auke Tellegen Science, Oct 12, 1990 v250 n4978 p223(6)
[4] Jensen, 1998, P. 178.
[5] “After the age of 10, the IQ score is essentially stable within the constraints of measurement error.” Herrnstein, R. J. & Murray, C. (1994). The Bell Curve: Intelligence and Class Structure in American Life. New York: Free Press
[6] Jensen has noted that the three areas in which there are true child prodigies are ones in which women do not excel. [American Renaissance, Vol 3, No. 8 August 1992, A Conversation With Arthur Jensen]
[7] W.YULE, R.D.GOLD & Carol BUSCH, 1982, Personality & Individual Differences 3.
[8] Diamond M. C., Scheibel, A. B., Murphy, Jr..G. M., & Harvey, T. (1985). The brain of a scientist: Albert Einstein. Experimental Neurology, 88, 198-204.
[9] Quoted by Edward M. Miller, Intelligence and Brain Myelination: A Hypothesis, Personality and Individual Differences, Vol 17, (December 1994) No. 6, 803-833.
[10] His Brain Measured Up By STEVEN PINKER, New York Times, June 24, 1999.
"People have long wondered about the source of genius. Last week three neuroscientists claimed to have found it using a pair of calipers. After examining the brain of Albert Einstein, they said it had large and unusually shaped inferior parietal lobules, a seat of mathematical and spatial reasoning.
The inferior lobule, or lower bulge of the lobe, in particular supports abstract mathematical and spatial reasoning. Presumably that is because the core of number sense is an intuition about spatial extent; people reason about numbers as if they were places along a line."
NEW SCIENTIST, 26 (June 1999). Raising Albert: Can studying dead brains ever tell us anything about genius?
"Just after his death in 1955, his brain was weighed, measured and photographed: its weight proved to be only average. Now, however, the photographs and measurements have been re-examined and his brain declared "exceptional". According to The Lancet (19 June) parts of his parietal lobe, an area of the brain associated with visuospatial abilities and mathematical ideas, are larger than normal and not divided by a fold in the usual way. The suggestion is that perhaps this allowed areas of his brain to develop more cross connections than usual."
[11] Jensen, A. R. (1980). Bias in Mental Testing, P. 83.
[12] The Society for Psychophysiological Research, Inc. Vol. 19, No. 4, Presidential Address, 1381
Research With Twins: The Concept of Emergenesis, David T. Lykken
[13] Jensen uses this word in his text The g Factor, so I use it here in this context.
[14] Jensen, 1998 (both quotes).
[15] Clearly Arthur Jensen. His contribution to psychometrics is the subject of Kings of Men: a Special Issue of the journal INTELLIGENCE about Arthur Jensen. This article can be easily located on the Internet with a search engine.
[16] Jensen, 1998, P. 367.
[17] Herrnstein, R. J. & Murray, C. (1994). P. 307.
[18] Jensen, 1998, P. 321.
[19] Rushton, J. P. (1999) Secular gains in IQ not related to the g factor and inbreeding depression unlike Black-White differences: A reply to Flynn. Personality and Individual Differences, 26: 381-389.
[20] Jensen, 1998, P. 322.
[21] SCIENTIFIC AMERICAN, November 1995, Volume 273, Number 5, Page 12.
[22] Rushton, J. P. (1999) Secular gains in IQ not related to the g factor and inbreeding depression unlike Black-White differences: A reply to Flynn. Personality and Individual Differences, 26: 381-389.
[23] Jensen, Arthur R. (2000) Some Recent Overlooked Research on the Scientific Basis of "The Bell Curve", Psycoloquy: 11,#106 Bell Curve (3).
[24] Jensen, Arthur R. (2000) Some Recent Overlooked Research on the Scientific Basis of "The Bell Curve", Psycoloquy: 11,#106 Bell Curve (3).
[25] Edward M. Miller, Intelligence and Brain Myelination: A Hypothesis, Personality and Individual Differences, Vol 17, (December 1994) No. 6, 803-833.