224 II. BIOLOGICAL  FOuNDATIONS
influences contribute to specific observed 
personality structures at distinct hierarchi -
cal levels (Krueger & De  Young, 2016). Even 
more informative would be finding func -
tional genetic variants at any level in the trait 
hierarchy and evaluating empirically the ex -
tent to which these variants correlate with 
other levels, including their association with 
specific brain systems. Given results of mo -
lecular genetics studies of personality (which 
have not identified highly specific functional 
genetic variants influencing personality; de 
Moor, Chapter 10, this volume), however, 
this is not likely to happen imminently.
Personality Stability over  Time Is 
Attributable More to  Genetic Than 
to Environmental  Influences
The multivariate approach to parsing genetic 
and environmental effects can be extended 
to study personality development over time 
in longitudinal studies. Perhaps not surpris -
ingly, this literature suggests that genes act 
more to maintain stability, whereas envi -
ronmental influences act more to promote 
change (Krueger, Johnson, & Kling, 2006). 
However, this general summary obscures 
ways in which developmental and social en -
vironments transact with genetic propensi -
ties (e.g., during the transition to adulthood; 
Hopwood et al., 2011). For example, early 
in life, emerging gene expression may be 
important in understanding temperamental 
development, tending to stabilize as matu -
rity proceeds. Plomin and colleagues (1993) 
examined change and continuity in tem -
perament in twins from 14 to 20 months of 
age and found evidence for newly expressed 
genetic influences at 20 months. Especially 
at life transition points such as emerging 
adulthood, parenthood, or major stressful 
events such as divorce, however, the new en -
vironmental circumstances these transitions 
present may initiate personality change (e.g., 
Hopwood et al., 2011). Later in life, the 
often relatively stable unique environmental 
niches people occupy, such as occupation, 
partner relationship, residence, and hobbies, 
may be important in understanding person -
ality continuity. We (Johnson, McGue, & 
Krueger, 2005) studied twins who were 59 
years old, on average, at a first assessment 
wave, and 64 years old, on average, at a sec -ond assessment wave. Consistent with other 
studies reviewed by Krueger and colleagues 
(2006), genetic effects on personality were 
essentially perfectly correlated across the 
two waves, emphasizing the role of genetic 
influences in personality stability. We were, 
however, the first to point out that non -
shared environmental influences on person -
ality were also highly correlated across the 
two waves (.53 to .73), and had been in pre -
vious such studies to date. Importantly, be -
cause nonshared environmental influences 
on personality were somewhat greater than 
50%, the relative contributions of environ -
mental and genetic influences to personality 
stability were essentially equal, because the 
nonshared environmental components in -
cluded error and greater proportions of total 
variance, consistent with other longitudinal 
studies of personality (Johnson et al., 2005).
“Environmental Measures” Are Partly 
Genetically Influenced and  Much of  the Genetic 
Influence on  These Measures Is Shared 
with Personality
One important and provocative finding 
that has emerged from behavioral genetics 
research is that many measures considered 
to reflect environmental circumstances are 
themselves shaped by genetics. For example, 
in his seminal work, David Rowe (1981, 
1983) showed that family experiences, such 
as adolescents’ perceptions of their parents’ 
acceptance and affection, are subject to ge -
netic influence. This indicates that our genes 
shape the way we experience the world. This 
likely happens through situation construal, 
but it also likely happens because genetically 
influenced patterns of behavior tend to elicit 
systematic patterns of responses from the en -
vironment, and because people gravitate to -
ward environments that meet their psycho -
logical needs and avoid environments that 
do not. In combination, all these processes 
act to intertwine nurture with nature (Plo -
min & Bergeman, 1991). This contradicts 
the assumption underlying all estimates of 
genetic and environmental influences that 
the two “sources” are independent. It does 
so in systematic ways that we understand 
clearly (Purcell, 2002). However, we do not 
know with clarity which of the many pos -
sible systematic ways apply to personality.