228 II. BIOLOGICAL  FOuNDATIONS
they show genetic influences that are closely 
related to those on personality. Nonshared 
environmental influences, manifested as 
anonymous components of variation, are 
as important as genetic influences, but their 
psychological natures remain elusive.
As we noted earlier in discussing non -
shared environmental influences, integrat -
ing these observations in a comprehensive 
theory of the origins of personality has 
proven challenging. Yet one often ignored 
assumption of the reviewed research may 
provide a key to advancing our understand -
ing and thereby integrating nature and nur -
ture. This assumption is that genes and en -
vironments are independent  as opposed to 
transactive . For example, researchers have 
traditionally estimated heritability as a sta -
tistic that applies to an entire population, to 
which is added an analogous statistic rep -
resenting environmental effects to account 
for the total variation—  as if the two were 
completely independent. We know of many 
examples, however, in both human and non -
human animals, in which genetic variance is 
not independent of environmental variance 
within populations. In particular, genetic 
variance can vary as a function of other 
variables. For example, in circumstances 
in which people are free to express genetic 
proclivities, genetic variation might be en -
hanced, whereas circumstances that con -
strain individual freedoms might dampen 
genetic variance.
Developments in statistical modeling have 
rendered these conceptual possibilities em -
pirically tractable. We provide only a brief 
description of these developments here (for 
more details, see Krueger & Tackett, 2007; 
Johnson, 2007). When genetic and environ -
mental influences on personality are esti -
mated using the traditional approach, esti -
mates are based on summary statistics that 
compare the overall similarity of pairs of 
people in specific groups, such as the over -
all similarity of MZ twins with the overall 
similarity of DZ twins. Newer models go 
well beyond this approach, because they do 
not model summary statistics, but rather 
individual-  level data. As a result, genetic and 
environmental influences on personality can 
be quantified as moderated by, or contingent 
on, other individual characteristics.
These modeling developments have 
opened up major conceptual possibilities for personality genetics that are just begin -
ning to be realized. A broad picture of how 
the genetic and environmental influences on 
personality are moderated by other charac -
teristics of persons—  and how personality 
can itself moderate the genetic and environ -
mental influences on other characteristics— 
is difficult to generate, because this research 
is in its infancy. Nevertheless, some exam -
ples serve to make the general point that 
genetic influences on personality seem to 
behave in a more nuanced manner than can 
be captured by classical variance partition -
ing approaches.
The major focus of this work has been 
on ways in which diverse aspects of family 
life—from relationships to family income— 
moderate genetic and environmental influ -
ences on personality. Characterizing genetic 
contributions to personality using overall 
heritability statistics entails summarizing 
across diverse family circumstances that 
may dampen or enhance genetic influences. 
This summarizing process may gloss over 
a range of scenarios, and looking beyond 
these statistics may be a key to understand -
ing how genetic and environmental influ -
ences transact within families to make fam -
ily members both similar and different from 
each other, as well as similar and different 
from people in other families (Collins, Mac -
coby, Steinberg, Hetherington, & Bornstein, 
2000). Rather than having direct and purely 
shared environmental influences that make 
people from the same families more similar 
than they would be based on genetic similar -
ity, family variables may sometimes act indi -
rectly, to moderate the impacts of genes and 
environments on personality.
For example, Boomsma, de Geus, van 
Baal, and Koopmans (1999) observed that 
religious upbringing reduced genetic vari -
ance in disinhibitory personality character -
istics. This would be consistent with the idea 
that genetic expression is suppressed in more 
restrictive environments. Jang, Dick, Wolf, 
Livesley and Paris (2005) observed that a 
variety of factors, such as parental bonding, 
family functioning, and nonassaultive trau -
matic events, impacted genetic and environ -
mental influences on emotional stability (the 
opposite of neuroticism), often by enhanc -
ing nonshared environmental influences. 
We (Johnson & Krueger, 2006) observed 
that nonshared environmental influences on 