594 CHAPTER 22Today, both biologists and many social scientists recognize that human behav-
iors are affected both by our genetic evolutionary heritage and by culture, the
product of the extraordinary human ability to think, learn, imagine, and speak.Variation in cognitive and behavioral traits
Because genetic variation is the basis of evolution of any characteristic, questions
immediately arise about the extent to which behavioral traits have a genetic foun-
dation. These questions apply both to variation among individuals and among
populations, and to supposedly “universal” human traits—“human nature.” Vari-
able traits can be analyzed by methods that partition the variance into genetic
and environmental components (see Chapters 6 and 15). Learning and culture
would contribute environmental variance. Among the traits studied this way,
two are especially controversial: cognitive abilities, or “intelligence,” and sexual
orientation.vARiATion in CoGniTivE ABiliTiES Cognitive abilities described as “intelli-
gence” are measured by IQ (“intelligence quotient”) scores. Psychologists dis-
tinguish various cognitive abilities (e.g., spatial ability, vocabulary), which con-
tribute to a “general” factor (g) that is analogous to an overall “size” factor that
captures correlated variation in different measurements on animal bodies (see
Appendix regarding principal components analysis). Genetic and environmental
components of variation can be difficult to distinguish in humans because family
members typically share not only genes but also environments. For this reason,
studies of people adopted as children are critically important. The genetic com-
ponent of variation is estimated by correlations between twins or other siblings
reared apart, or by adoptees’ correlations with their biological parents. Because
adoption agencies often place children in homes that are similar to those of their
siblings, many modern studies try to measure such environmental correlations
and take them into account. The environmental component of variance (VE) can
then be broken down into the variance due to shared environment and a resid-
ual environmental variance due to other, unmeasured influences. Twins have
been important in human genetic studies, since monozygotic (“identical”) twins
should be more similar than dizygotic (“nonidentical”) twins if variation has a
genetic component. In addition to these correlational studies, researchers are
starting to use genome-wide association studies (GWAS) to find genetic markers
that are correlated with differences in IQ scores (e.g., [64]).
Several conclusions have arisen from many such studies [15, 82, 93]. First, gen-
eral cognitive ability (g) has quite high heritability (h^2 , or G/P, see Chapter 6). For
example, h^2 was estimated as 0.70 in a study of 11-year-olds in Scotland and as
0.67 in 11-year-olds in Minnesota, and the variance ascribed to shared environ-
ments was 0.21 and 0.26, respectively, in the two studies. In these studies, IQ has
about the same heritability as height. Second, in these and many other studies,
h^2 increases with age: the effects of childhood environment are eroded over time.
Third, the various cognitive abilities that have been compiled into a general g fac-
tor are strongly correlated with each other, with genetic correlations of about 0.60.
As we know from studies of phenotypic plasticity, a genetic basis for a trait does
not mean that the trait is fixed or unalterable. Twin studies have suggested that
the heritability of human height is 0.8 or more, yet in many industrial nations,
mean height has increased considerably within one or two generations as a result
of nutritional and other improvements. Similarly, IQ scores are increased greatly in
children who have been adopted into homes that provide a richer, more stimulat-
ing learning environment [82]. By the same token, high heritability of variation22_EVOL4E_CH22.indd 594 3/22/17 1:49 PM