tent on their physical appearance. Social skills and confidence, in turn, may in-
fluence how one performs on IQ tests. The result would be that identical twins,
even when reared apart, will tend to perform similarly on IQ tests, yet the
similarity in performance has nothing to do with their intellectual potential.
Instead, it has to do with their physical appearance. It could be that one twin,
should she or he grow up in an environment that downplays physical appear-
ance, might obtain a very different IQ score than the other twin.
There are other hypothetical examples I could work out. Maybe, for example,
the similarity in IQ between twins reared apart is due to similarity of their
metabolic rates, or to their resistance to diseases, or to any of a number of other
factors that may be genetically inherited and indirectly affect performance on
IQ tests. The point of these hypothetical examples is to show that establishing
that twins reared apart perform similarly on IQ tests does not necessarily prove
that there is a direct genetic basis for intellectual performance. Incidentally, the
sameargumentcanbemadewithrespecttothehigherIQcorrelationbetween
the biological parents and their children whom they do not raise than between
theadoptiveparentsandthosesamechildren.Someofthegenestheadopted
children inherit from their biological parents influence their scores on IQ tests,
but it remains unclear what aspect of biology those inherited genes control.
The Role of Environmental Factors in Intellectual Differences
One of the unfortunate implications sometimes drawn from the hereditarian
theory of intelligence is that environmental factors are likely to have a rather
meager effect on intelligence. Consequently, it is not worth spending money
and effort trying to improve substantially the intelligence of people who might
seem ‘‘intellectually at risk.’’ Now, strictly speaking, one need not draw this
implication from hereditary theory, because hereditarians concede that the en-
vironment can have an impact on intellectual development. But the problem is
that an emphasis on the genetic basis of intellectual differences can blind one to
the possibility that environmental factors may have a rather potent effect on
intelligence. Genetically based differences lead to the idea of inevitable differ-
ences (Gould, 1981). Yet a variety of studies have demonstrated that environ-
mental intervention can substantially improve intellectual capabilities.
Family and School Environments Some studies have looked at the behaviors of
parents to see which are correlated with their children’s intellectual compe-
tence. For example, the parents’ use of language correlates with their children’s
performance on IQ tests (Hart & Risley, 1992). Child-rearing practices also cor-
relate with the child’s intellect. White (1978), for example, found that parents
who reared intellectually competent children tended to do three things: first,
they provided a structured, safe, and interesting physical environment for their
children. Second, they spent a lot of time helping their children solve problems.
Third, they established and enforced clear-cut rules, but in a loving and re-
spectful manner.
Such studies suggest the importance of parenting styles in the acquisition
of intellectual competence. The hereditarian could, however, still argue that is
the favorable genes of the parents that lead them to use reasonable parenting
techniques, and that the intellectual competence of their children is mainly a
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