 9. Behavioral Genetics and Personality  235
that personality structure is so richly hier -
archical. In addition, regardless of the level 
of detail at which we measure personality, 
the traits we measure are amalgamations of 
different motivations and responses. For ex -
ample, compliance, a facet of agreeableness 
in the five-  factor model, may involve percep -
tion that cooperation has some long-term 
reward, fear of punishment, actual enjoy -
ment of the activities involved for their own 
sakes, lack of alternative activities and intol -
erance for boredom, and, in any case, abil -
ity to comprehend requirements and activate 
appropriate behaviors. Activity, a facet of 
theoretically independent extraversion in the 
five- factor model, may involve many similar 
motivations and responses to varying or 
even similar degrees. Obviously, experience 
will affect how one perceives one’s situation 
with respect to these kinds of motivations 
and responses as well, but the point here is 
that it is very unlikely that particular genes 
will be involved in activity but not in compli -
ance, or vice versa.
Universality of at least some level of ex -
pression of all basic personality traits sug -
gests that variation in genetic expression 
may underlie much of the observed varia -
tion in personality traits. Though genetic 
expression itself is at least partially under 
genetic control (York et al., 2005), work 
with experimental nonhuman animals 
makes clear that variation in genetic expres -
sion can also be triggered by environmental 
circumstances. For example, Weaver and 
colleagues (2004) described differential en -
vironmental effects on genetic expression in 
rats. Differences in maternal licking, groom -
ing, and nursing practices were associated 
with long-term differences in offspring’s 
hypothalamic–  pituitary–  adrenal response 
to stress. These differences appeared to re -
sult from differences in DNA methylation 
of a glucocorticoid receptor gene promoter 
in the hippocampus. The differences in ma -
ternal treatment appeared to contribute to 
parental behavior by the offspring as well, 
leading to differences in stress response that 
were transmitted from generation to gen -
eration. Whether or not there are processes 
directly analogous to these in humans, it is 
highly likely that these kinds of general pro -
cesses take place in humans, contributing to 
genetic variation that we can sometimes pick up through statistical decomposition of vari -
ance but not through examination of DNA 
samples.
In humans, MZ twins provide informa -
tion about another mechanism that compli -
cates the search for genes for polygenic traits 
such as personality. Though MZ twins share 
common genetic backgrounds that tend to 
make them similar, significant variation in 
gene expression remains. Extent of this vari -
ation has been observed to increase with age 
(Fraga et al., 2005), suggesting that it is sub -
ject to environmental influences. But post -
natal environmental experiences are prob -
ably not the only sources of these epigenetic 
differences. MZ twins tend to be similar to 
similar degrees regardless of whether they 
are reared together or apart (Wong, Gottes -
man, & Petronis, 2005). This “similarity of 
similarity” may be due to rGE. It may also 
result from tendencies for additional resem -
blance due to shared rearing environments 
to be offset by differentiation, as described 
earlier.
Two Broad Questions That Remain Relevant 
to Genetic Research on Personality
We began this chapter with well-  established 
observations in personality genetics and 
moved to current trends in research. In clos -
ing, we briefly outline two very challenging— 
but, in our view, perennially important— 
questions that we feel are highly relevant in 
contemplating the link between genetics and 
personality.
Do We Need to Find Specific Genes 
and Specific Environmental Influences 
to Understand Personality  Processes?
For many, the primary purpose of demon -
strating the presence of genetic influences 
on human personality and other behavioral 
traits has been to provide evidence warrant -
ing a search for the specific genes involved. 
Though the fact that genes code for proteins 
rather than psychological characteristics 
is well established, the implications of this 
fact for understanding behaviors influenced 
by many genes have not been fully absorbed 
(Kendler, Kuhn, Vittum, Prescott, & Riley, 
2005). In particular, we cannot expect that 