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 Genetic
research has made important discoveries about intelligence during the
past few decades. To outline some of these findings, I won’t spend
space on the measurement of intelligence except to say that what I mean
by intelligence is general cognitive ability defined as g. All reliable
and valid tests of cognitive ability intercorrelate at a modest level—g
is what they have in common. g is often assessed as a total score across
diverse cognitive tests as in intelligence (IQ) tests, although it is
more accurately indexed by an unrotated principal component that best
reflects what is in common among the tests. Nearly all genetic data have
been obtained using measures developed from this psychometric perspective,
primarily IQ tests. One new direction for genetic research on intelligence
is to investigate other measures such as information-processing and more
direct measures of brain function such as evoked potentials, positron
emission tomographic scans, and functional magnetic resonance imaging
and to explain how these measures relate to g.
g
clearly runs in families. The correlations for first-degree relatives
living together average 0.43 for more than 8,000 parent–offspring
pairs and 0.47 for more than 25,000 pairs of siblings. However, g might
run in families for reasons of nurture or of nature. In studies involving
more than 10,000 pairs of twins, the average g correlations are 0.85 for
identical twins and 0.60 for same-sex fraternal twins. These twin data
suggest a genetic effect size (heritability) that explains about half
of the total variance in g scores.
Adoption
studies also yield estimates of substantial heritability. For example,
identical twins reared apart are almost as similar for g as identical
twins reared together. Adoption studies of other first-degree relatives
also indicate substantial heritability, as illustrated below by recent
results from the Colorado Adoption Project (CAP). Model-fitting analyses
based on dozens of adoption and twin studies estimate that about half
of the total variance can be attributed to genetic factors. Genetic influence
on g is not only statistically significant, it is also substantial, especially
when compared to other research in the behavioral sciences that rarely
explains 5% of the variance. Genetic research has moved beyond the question
of heritability of intelligence to investigate developmental changes,
multivariate relations among cognitive abilities, and specific genes responsible
for the heritability of g. These 3 issues will now be addressed.
When
Francis Galton first studied twins in 1876, he investigated the extent
to which the similarity of twins changes over the course of development.
Other early twin studies of g were also developmental, but this developmental
perspective faded from genetic research until recent years. One of the
most interesting findings about g is that heritability increases steadily
from infancy (20%) to childhood (40%) to adulthood (60%). For example,
a recent study of twins aged 80 years and older reported a heritability
of about 60. (Fig. 1)
The
20-year longitudinal CAP confirms this finding using the adoption design.
CAP is a 25-year study of 245 children separated from their biological
parents at birth and adopted in the first month of life. Correlations
are shown between g scores of the biological parents and their adopted-away
children, the adoptive parents and their adopted children, and nonadoptive
or control parents and their children matched to the adoptive families.
Correlations between nonadoptive parents and children increase from less
than 0.20 in infancy to about 0.20 in middle childhood and to about 0.30
in adolescence. The correlations between biological mothers and their
adopted-away children follow a similar pattern, indicating that parent–offspring
resemblance for g is due to genetic factors. In contrast, parent–offspring
correlations for adoptive parents and their adopted children hover around
zero, which suggests that family environment shared by parents and offspring
does not contribute importantly to parent–offspring resemblance for
g.
Why
does heritability of g increase during the life span? Perhaps completely
new genes come to affect g as more sophisticated cognitive processes develop.
A more likely possibility is that relatively small genetic effects early
in life snowball during development, creating larger and larger phenotypic
effects, perhaps as individuals select or create environments that foster
their genetic propensities.
There
is more, however, to cognitive abilities than g. In the widely accepted
hierarchical model of cognitive abilities, specific cognitive abilities
include components such as spatial, verbal, speed-of-processing, and memory
abilities, each indexed by what is in common among several tests of each
ability. Less is known about the genetic and environmental origins of
individual differences in specific cognitive abilities, but they also
appear to show substantial genetic influence, although less than g.
A
surprising finding concerning specific cognitive abilities is that multivariate
genetic analyses indicate that the same genetic factors largely influence
different abilities. What this finding means concretely is that if a specific
gene were found that is associated with verbal ability, the gene would
also be expected to be associated with spatial ability and other specific
cognitive abilities. This finding is surprising because it goes against
the tide of the popular modular theory of cognitive neuroscience that
assumes that cognitive processes are specific and relatively independent
of one another. The multivariate genetic results are consistent with a
top-down model in which genetic effects of g pervade a broad range of
cognitive processes. An even more surprising finding in 4 out of 4 studies
is that genetic effects on measures of school achievement overlap almost
completely with genetic effects on g. The converse of this finding of
genetic overlap is equally interesting. Although genetics accounts for
the overlap between school achievement and g, discrepancies between school
achievement and g, often used to describe underachievers, are largely
environmental in origin.
Heritability
of complex dimensions such as g seems likely to be due to multiple genes
of varying but small effect size rather than a single gene that has a
major effect. Genes in such multiple-gene systems are called quantitative
trait loci (QTLs).
Unlike
single-gene effects like PKU that are necessary and sufficient for the
development of a disorder, QTLs contribute interchangeably and additively
like probabilistic risk factors. Traditional methods for identifying single-gene
effects are unlikely to succeed in identifying QTLs.
A QTL study applying new genetic approaches
to g yielded a replicated association in a study comparing groups of children
of high g and children of average g. The gene is insulin-like growth factor-2
receptor (IGF2R) on chromosome 6, which has recently been shown to be
especially active in brain regions most involved in learning and memory.
The frequency of one of the alleles was twice as high in 2 groups of children
with high g compared with 2 groups of children with average g (about 30%
versus 15%).
Identifying
replicable QTLs associated with g will make it possible to address questions
about development, differential diagnosis, and gene–environment interplay
through the use of measured genotypes rather than indirect inferences
about heritable influence based on familial resemblance. Such QTLs will
also provide discrete windows through which to view neurophysiological
pathways between genes and cognitive development. As is the case with
most important advances, identifying genes for cognitive abilities and
disabilities will also raise new ethical issues. These concerns must be
taken seriously, but they are based largely on misconceptions about genetic
research on complex traits that are influenced by multiple genes as well
as multiple environmental factors.
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