reared apart; and there is a significant correlation
between the IQs of nonbiologically related but
reared-together relatives”(pp. 60–61).
Even if heritability estimates were 100%, this
does not rule out the possibility that IQ scores may
change. Some“genetically determined”traits, such
as height, can be influenced by environmental cir-
cumstances, and genetic disorders can be controlled
or even cured by environmental intervention. In
fact, heritability of intelligence does not appear to
be stable across the life span; it ranges from about
20% in infancy, to 60% in young adulthood, to
80% in old age (Deary et al., 2010; Petrill, 2005).
This indicates that the environment plays a
greater role in intelligence in children than in
adults, raising the possibility that interventions at
younger ages might be able to increase IQ scores.
Although some short-term gains or changes have
been noted, in general the research examining the
efficacy of psychosocial interventions for improving
IQ scores has been mixed (Kamphaus, 1993;
Neisser et al., 1996). Despite these sometimes dis-
appointing results, the general notion that favorable
environments should be provided so as to allow
individuals to realize their innate potential seems
both plausible and a worthy goal (see Box 7-3).BOX7-3 The Concept of HeritabilityThe concept of evergreens in A varies in height. The
degree to which they vary from each other is called the
variance. What produces this variance? Some is proba-
bly due to genetics. To find out how much, we equate
environmental conditions such as soil, water, and sun-
light (indicated by different shadings of the ground).
We now take a random group of seedlings chosen
from A. We plant them in this equated environment
(B) and wait patiently until they mature. We note thatthe size variation in B is less than it was in A. This
reflects the fact that environmental conditions in B are
equal for all the trees so that any environmental
sources of variance have been eliminated. The remain-
ing variance in B is entirely produced by genetic fac-
tors. Therefore, the heritability of height for A is the
variance in B (the variation attributable to genetic
factors) divided by the variance in A (the total variation
in the population).AB
SOURCE: Adapted fromPsychology, by Henry Gleitman, by permission of W. W. Norton & Company, Inc. Copyright © 1981 by W. W. Norton &
Company, Inc.204 CHAPTER 7