EvoluTion And SoCiETy 595
within populations does not mean that differences among populations have a
genetic basis, because the populations may have very different environments. This
is important for interpreting the differences in average IQ scores among so-called
“racial” or ethnic groups, such as the 15 points (1 standard deviation) that, until
recently, separated the average scores of European-Americans and African-Amer-
icans. This difference is due to the populations’ very different social, economic,
and educational environments [74, 81]. That gap has been reduced by one-third in
recent years, and studies of adopted children show that black and white children
reared in the same environment have similar IQ scores [82]. One study found that
the average IQ of German children fathered by white American soldiers during
World War II was nearly identical to the IQ of those with black American fathers.
All the evidence says that people of different ethnicities do not differ genetically in
cognitive ability.vARiATion in SEXuAl oRiEnTATion Humans exhibit a bimodal continuum
of sexual orientation, from exclusively heterosexual through bisexual to exclu-
sively homosexual. (Sexual orientation has been studied much more in men
than women.) At least a small percentage of men in almost all cultures stud-
ied, throughout the world and throughout history, have expressed homosexual-
ity, and in some cultures this has been the norm [39]. It seems likely, then, that
sexual orientation has been variable since before our species spread throughout
the world. Homosexual behavior has been recorded in diverse animals, includ-
ing more than 100 species of mammals, and is a subject of increasing attention
among evolutionary biologists [95, 107]. Sexual orientation is not chosen, nor
determined by childhood experience [121]; it has a largely biological basis, and
has a heritability of about 0.2–0.4 [61, 68]. A genomic scan found two chromo-
some regions associated with male sexual orientation, one of which had been
tentatively identified in an earlier study [102]. However, several lines of evidence
suggest that homosexuality may not be based in DNA sequence, but might result
if epigenetic marks that canalize sexual development in one sex became inherited
by the opposite sex [97]. Epigenetic marks that influence a fetus’s response to
testosterone might “feminize” certain pathways in the brains of males, or “mas-
culinize” those of females, and result in same-sex orientation.
To the extent that homosexual orientation has a genetic basis, the rather high
frequency of homosexuality is evolutionarily enigmatic, since homosexuals are
generally supposed to reproduce less than heterosexuals. (However, there is little
evidence that this has been true for most societies throughout human history;
even today, social expectations cause many homosexual people to marry and have
children.) Assuming that male homosexuals do have a low average reproductive
rate, several population genetic models could account for a stable polymorphism
in sexual orientation [22]. The models that match the data best ascribe homosexual
inclination to at least two loci, including at least one on the X chromosome, with
effects such that the reproductive disadvantage of male homosexuality is balanced
by the increased fecundity of females with the same X-linked allele. This situa-
tion would be an example of a polymorphism maintained [by sexually antagonis-
tic selection (see Chapter 12)]. In northern Italy, gay men reported a significantly
higher proportion of homosexual maternal relatives (5 percent) than paternal
relatives (2 percent), consistent with inheritance on the X chromosome. Moreover,
both the mothers and maternal aunts of gay men had significantly more children
than those of heterosexual men (FIGURE 22.7), and had greater reproductive health
[22, 24]. Genetic factors inclining men toward homosexual orientation may provide
a reproductive advantage to women.22_EVOL4E_CH22.indd 595 3/22/17 1:49 PM