Recent physiological research has suggested correlations between perfor-
mance on intelligence tests and physiological measures such as cortical meta-
bolic rate or neural conduction speed. Usually, these neurophysiologically
based models implicitly suppose that intelligence is unitary—that some aspect
of neurophysiology that permeates all intellectual tasks is the factor that gives
rise to individual differences in cognition. While intriguing, such research has
not yet elucidated the underlying biological mechanisms or the causes of such
correlations.
At any rate, correlations among IQ subtests or between IQ tests and aca-
demic success can be explained without supposing that all intellectual dif-
ferences represent differences in a single underlying substrate of the various
cognitive systems. In section 36.3 I discuss how the correlations could reflect
motivation or the limited range of skills measured by IQ tests and taught in
schools. Indeed, if one examines creativity, social skills, or practical skills used
in everyday life, the correlations between such skills and IQ tests are essentially
nonexistent.
One alternative to the unitary model is the claim that there are several dis-
tinct, relatively autonomous categories of intelligence. Howard Gardner (1983),
for example, claims that there are six different categories of intelligence, and
cites physiological and anthropological evidence to bolster his claim. Another
alternative claims that there are potentially an unlimited number of categories
of intelligence, any one of which may be measured in a potentially unlimited
number of ways. The ways a culture defines and measures intelligence reflect
the values and goals of the culture, and not something intrinsic to the biology
of people.
In section 36.4 I discuss the hereditarian claim that intelligence is largely ge-
netically determined. The claim is supported by familial correlations in IQ per-
formance, and by the high correlation between the IQ scores of identical twins
reared apart. However, the familial pattern of correlations is also consistent
with a substantial impact of environmental factors on intelligence. The twins-
reared-apart findings only show that some genetically determined biological
mechanism underlies IQ performance. That mechanism might control intellec-
tual processes, but it might also control physical appearance, metabolic rate,
resistance to disease and/or any of a number of other traits.
One unfortunate implication sometimes drawn from a theory that empha-
sizes the genetic basis of intelligence is that environmental factors are likely to
have minimal influence on intellectual development. In fact, though, a variety
of studies demonstrate that appropriate environmental intervention can im-
prove the intellectual performance of individuals who might otherwise be at
‘‘intellectual risk.’’ Furthermore, performance on IQ tests is rising about 3 IQ
points a decade all around the world.
Another unfortunate implication historically drawn by hereditarians is that
ethnic differences in IQ performance reflect genetic differences among racial
and other ethnic groups. However, adoption studies and other research con-
vincingly makes the case that differences among the average IQ scores of ethnic
groups reflect environmental and cultural differences among groups. If mem-
bers from two different ethnic groups are raised in similar circumstances, their
808 R. Kim Guenther