230 II. BIOLOGICAL  FOuNDATIONS
related example, Caspi and colleagues (2002) 
observed that boys carrying the low-  activity 
variant of the monoamine oxidase A (MAO-
A) gene were more likely to display antisocial 
behavior in adolescence and young adult -
hood, but only if they had been exposed to 
severe parental maltreatment (see also Kim-
Cohen et al., 2006). This type of candidate 
gene × environment research spawned many 
replication attempts. Although the ideas 
driving this approach have garnered much 
attention, specific candidate gene × environ -
ment effects have been difficult to replicate 
(Dick et al., 2015). This does not necessar -
ily mean that such effects are not operating, 
as none of the interactions reported has in -
volved more than modest amounts of vari -
ance, and there is no theoretical reason that 
such small interactive influences of specific 
genes should be expected to be present in all 
populations and environments.
Gene–  environment correlations ( rGE) 
occur when genetic differences are associ -
ated with differential exposure to environ -
mental circumstances. Again, genes involved 
in skin color offer a simple example when 
people with lighter skin restrict exposure to 
sunlight to avoid sunburn. As a personality- 
related example, Jaffee and colleagues 
(2004) observed that 25% of the variance in 
the corporal punishment children received at 
the hands of their parents could be attrib -
uted to genetic influences on the children’s 
own misbehavior, suggesting that parents 
were punishing children more genetically 
prone to misbehave more than children less 
genetically prone to do so.
Though the behavioral genetics models 
that assumed independence were a first step 
toward understanding genetic and environ -
mental influences, it is becoming increas -
ingly clear that G × E and rGE are likely to 
be common and to be useful in understand -
ing how nature and nurture transact. The 
efforts people undertake to seek or create 
environments compatible with their genetic 
tendencies are fundamental to the process 
of evolution, and differences in response to 
the environment caused by genetic differ -
ences are the raw material for natural selec -
tion (Ridley, 2003). We may tend to think of 
evolution as a long-term process that took 
place in the long-  distant past, but the day-
to-day behavioral activities of response and 
adaptation to the environment—  activities in which we all engage every day—are the stuff 
of which it is made. Behavior occurs through 
genetic expression, and genetic expression 
reflects the environments in which it takes 
place. There is a large body of evidence 
for genetic influences on all other areas of 
human biological, psychological, and be -
havioral functioning; it would be distinctly 
strange if there were no genetic influences on 
selection of, and responsiveness to, environ -
mental circumstances.
Importantly, violations of the “indepen -
dence assumption” do not invalidate tradi -
tional behavioral genetics methods, though 
they do bias them in systematic ways. Addi -
tionally, they render the estimates applicable 
only on overall, average bases. In the presence 
of G × E or rGE, the components of variance 
attributable to genetic and environmental 
influences are not static within the popu -
lation: Genetic variance could be relatively 
large within one segment of the population 
and relatively small in another, and the same 
is true for environmental variance (John -
son, 2007). The estimates are specific to the 
population in which they are developed for 
essentially the same reason: The differences 
in the components of variance may depend 
on other characteristics of the individuals in 
the population, characteristics that can vary 
from population to population, and within 
populations over time. These characteristics 
can be measured either categorically or con -
tinuously. For example, Rose, Dick, Viken, 
and Kaprio (2001) observed that genetic in -
fluences are more important in explaining 
alcohol-  use patterns in adolescents residing 
in urban areas, whereas shared environmen -
tal influences are more important for adoles -
cents residing in rural areas. We (Johnson & 
Krueger, 2005) found that genetic influences 
on physical health are continuously smaller, 
with continuously greater perceived control 
over life. Uncovering how these processes 
are involved in development and manifes -
tation of personality means identifying rel -
evant moderating variables and measuring 
them accurately (Rutter, Moffitt, & Caspi, 
2006), along with considering the possibility 
that many variables could have moderating 
influences in different directions, making it 
necessary to understand how they come to -
gether.
G × E and rGE introduce systematic dis -
tortions, or bias, in estimates of genetic and 