have criminal convictions have no higher rate of criminal behavior
than other adoptees unlesstheir biological parents also have crimi-
nal records—that is, unless the children are at genetic risk (Plomin
et al., 2001). This is called gene-environment interaction.In like man-
ner, the same vocational encouragement (say, toward Realistic inter-
ests) will bear most fruit with the genetically most receptive.
6.Age-related increase in the heritability of (at least some) general
traits. Perhaps the greatest surprise for behavioral geneticists has
been their discovery that the heritability of intelligence rises from 20
percent in infancy, to 40 percent in early childhood, to 60 percent
in adolescence, and then to 80 percent in late adulthood. In other
words, differences in developed intelligence come to more closely
reflect underlying genetic differences as people age. Less definitive
evidence suggests that the heritability of its close correlates (for
example, academic achievement), broad mental abilities (for ex-
ample, verbal and spatial abilities), and personality disorders (for
example, antisocial personality) also rise with age, whereas shared
environmental effects wane (Plomin et al., 2001; shared environ-
mental effects will be discussed next). This unexpected age trend in
heritabilities has prompted behavioral geneticists to develop new
theories of child and adult development that, as will be described
later, emphasize people’s lifelong efforts to find environments that
are compatible with and reinforce their genetic tendencies.
7.Importance of nonshared, rather than shared, environmental
influences for the development of general traits. There are two types
of nongenetic influences on development: shared and nonshared en-
vironmental effects. Sharedinfluences, by definition, result from
environments that family members experience in common (fam-
ily income, schools, and so on) and that make siblings more alike.
Nonsharedinfluences are ones that affect one family member but
not another (say, illness) and that make siblings lessalike. Scien-
tists had long assumed that shared influences have large, lasting
influences on general traits such as intelligence, but research has
strongly suggested otherwise. Shared environments turn out notto
create similarity among siblings in either mental ability (except
GOTTFREDSON’S THEORY OF CIRCUMSCRIPTION, COMPROMISE, AND SELF-CREATION 113