work shows that among Canadian Eskimos, a culture in which both males and
females travel far from home and hunt, there are no differences in spatial abili-
ties between males and females (Berry, 1966).
Indeed, at present in our culture, it is at least debatable whether there are any
reliable male–female differences in verbal and math skills. Hyde and Linn
(1986, 1988) reviewed 165 studies of verbal ability representing over 1.4 million
people and found no average difference between males and females. Moreover,
Hyde, Fennema, and Lamon (1990) reviewed 100 studies of mathematical per-
formance and found that sex differences were quite small, but tended to favor
females in large samples that are taken from the general population. It is only
in the population of mathematically gifted individuals that males outperform
females, on the average.
Carol Tavris, in her splendid bookThe Mismeasure of Woman(Tavris, 1992),
reviews evidence that suggests that male and female brains learn, reason, and
process information in similar ways. Tavris also discusses the bias against pub-
lishing research that finds no sex differences in cognition, and the unfortunate
consequences this bias has for women. For example, a belief that males have
superior mathematical skills, sustained by a bias against publishing studies that
show no sex differences in mathematical skill, provides a rationale for exclud-
ing women from the sciences and for denigrating the few women who do
manage to become scientists.
Conclusions about the Genetic Basis of Intelligence
There seems to be no easy way to summarize the evidence relevant to the ge-
netic basis for intelligence. Because we are unable to conduct controlled ex-
periments that vary genes and environments, we remain ignorant of how to
interpret correlations in the IQ scores of individuals who share genes. Indi-
viduals who share genes almost always share environments. With regard to
sex differences in cognition, it is difficult to disentangle the influence of sex-
linked genes and sex-linked environments. It is true that the twins-reared-apart
studies, as well as other research on adoption, suggest that something that is
genetically inherited causes differences in scores on IQ tests. However, it is not
clear what genetically controlled biological mechanism is responsible for the
similarity in IQ scores. Indeed, at this point we do not really know what bio-
logical mechanisms are the underlying basis for individual differences in any of
the potentially limitless kinds of skills a person can acquire. All we can say
with certainty is that the biological mechanisms underlying intellectual devel-
opment are, especially in our species, designed to enable us to learn from the
environment. Consequently, any act of the intellect will invariably reflect both
biological and environmental factors. Genetic models of intellectual differences
to date lack any clear explanation of what biological mechanisms underlie in-
dividual differences. Sex hormones may be a basis for male–female differences
in cognition; however, it is possible that sex hormone production may be the
effect of different environments and not necessarily the direct cause of cognitive
differences.
Any useful model needs to explain how a genetically determined biological
mechanism interacts with various aspects of the environment to produce in-
tellectual development. It seems pointless to argue about whether intellectual
806 R. Kim Guenther