development is primarily determined by the genes or by the environment, be-
cause either can dominate depending on the circumstances. If people are given
no exposure to music, for example, they will not develop musical skill. If
people are born deaf as a result of a genetic defect, they will not develop any
musical skill.
And, of course, the role of genes and the biological mechanisms controlling
intellectual differences is invariably complicated by the difficulty in defining
and measuring intelligence. As I suggested in earlier sections of this chapter,
a good case can be made that there are a potentially vast number of relatively
autonomous skills that a person can acquire, any one of which could be as-
sessed in many different ways. The effects of genetically controlled biological
mechanisms and environmental variables could be quite different depending
on what aspect of intelligence one cares to study.
My own sense is that the influence of genes and environmental variables is so
complex and intertwined, the research limitations on the effects of genes so in-
tractable, and the notion of intelligence so potentially multifaceted, that it is not
possible to know exactly how genes and environmental variables interact to
produce individual differences in cognition. This need not be a distressing state
of affairs, however. Our goal as psychologists and educators should be to try to
create the best possible environments for fostering the acquisition of intellectual
competence in our children, regardless of their genetic makeup.
Summary and Conclusions
The integrating theme for this chapter was a contrast between a hereditarian
approach to individual differences in intelligence and a multi-faceted approach.
The hereditarian approach make two essential claims: intelligence is unitary
and is determined primarily by the genes one inherits. The multi-faceted ap-
proach claims that there are many different and relatively autonomous do-
mains of intelligence. Intellectual skill in one domain is typically unrelated to
intellectual skill in other domains.
In the first section, I discussed the rise of the hereditarian approach to intel-
ligence and the intelligence testing movement. Probably the most historically
significant event in the history of intelligence testing was the development of
IQ tests. IQ tests are known to be moderately correlated with grades in school,
occupational status, and success in an occupation.
In section 36.2, I discussed the main evidence for a unitary view of intelli-
gence, which is that performance on the subtests that make up the IQ inventory
and between IQ scores and academic achievement are positively correlated. A
generic information processing perspective proposes that intellectual tasks are
performed by a common information processing system. Differences in intel-
lectual capability are due to the speed and efficiency with which various stages
of the system are executed. One line of evidence in support of the information
processing perspective comes from research that shows that the shorter the
stimulus exposure time at which people can accurately discriminate between
the length of two lines, the higher the person’s IQ score. In a sense, the rise of
the information processing analysis of individual differences represents a re-
emergence of the ideas of Francis Galton, who espoused them about 100 years
ago.
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