 9. Behavioral Genetics and Personality  231
environmental influences from traditional 
models. These distortions have different ef -
fects, depending on the nature of the inter -
action or correlation that violates the inde -
pendence assumption. Specifically, in twin 
studies, G × E between genetic and shared 
environmental influences acts to bias the es -
timate of genetic influence high, whereas G 
× E between genetic and nonshared environ -
mental influences acts to bias the estimate 
of nonshared environmental influence high; 
rGE between genetic and shared environ -
mental influences acts to bias the estimate 
of shared environmental influence high; and 
rGE between genetic and nonshared environ -
mental influences acts to bias the estimate of 
genetic influence high (Purcell, 2002).
These principles can be used in combina -
tion with the typical results of behavioral 
genetic studies of personality to pinpoint 
likely mechanisms of G × E and rGE involv -
ing personality. G × E between genetic and 
shared environmental influences may have 
exaggerated the apparent genetic influences 
on personality at the expense of the shared 
environmental influences typically absent in 
study results. In contrast, rGE between ge -
netic and shared environmental influences 
is unlikely, as it would increase apparent 
shared environmental influences on person -
ality, and we typically observe none. This 
means, for example, that we might expect 
to find neighborhood effects on neuroti -
cism that differ by genetic vulnerability to 
neuroticism, but we would not expect to 
find that family culture involving neuroti -
cism influences how people make residential 
choices. G × E and rGE involving genetic and 
nonshared environmental influences have 
offsetting effects and may be common. They 
both involve transactions between genetic 
influences and environmental circumstances 
unique to each individual. Thus, even rGE be-
tween genetic and nonshared environmental 
influences, which tends to inflate estimates 
of genetic influence, will tend to be idiosyn -
cratic in form (e.g., individual differences in 
responses to trauma).
Models that estimate genetic and envi -
ronmental influences can contribute im -
portantly to amplification and illustration 
of theoretical principles of personality de -
velopment. In particular, Caspi, Roberts, 
and Shiner (2005) have articulated the cu -
mulative continuity and the coresponsive principles of personality development. The 
cumulative continuity principle  states that 
personality stability increases with age. Sta -
bility peaks perhaps around age 60, though 
personality characteristics are never com -
pletely fixed. Genetic influences contribute 
to this stability in a straightforward way 
(Johnson et al., 2005; Pedersen & Reynolds, 
1998; Viken, Rose, & Koskenvuo, 1994). 
We know this because the same genetic in -
fluences contribute to personality at earlier 
and later adult ages. However, as described 
earlier, straightforward nonshared environ -
mental influences also contribute to stability 
in personality (Johnson et al., 2005). This 
finding likely reflects the fact that, for many 
people, many aspects of the environment, 
such as place of residence, spousal relation -
ship, occupation, and leisure and social ac -
tivities, remain relatively stable over long pe -
riods of time in adulthood (Johnson, 2007).
In addition to these contributions of 
straightforward genetic and nonshared en -
vironmental influences to personality sta -
bility, transactions between genetic and en -
vironmental influences such as G × E and 
rGE also make contributions. Caspi and 
colleagues (2005) have identified niche- 
building processes, in which individuals cre -
ate, seek out, or end up in environments that 
are correlated with their personality traits, 
as fundamental to personality stability. Be -
cause of the genetic influences on personal -
ity traits, these niche-  building processes are 
perfect examples of rGE in action. In addi -
tion, as Caspi and colleagues noted, once 
people enter trait-  correlated environments, 
those environments may have causal effects 
of their own, contributing to maintenance of 
the personality trait and preventing develop -
ment of opportunities for change. Such ef -
fects are often examples of G × E.
Niche construction processes are an active 
area of interest in evolutionary research, and 
these ideas may also be useful in personal -
ity psychology (Laland, Matthews, & Feld -
man, 2016; Penke & Jokela, 2016). Con -
sider, for example, the following two studies 
that touched on these issues in personality 
psychology, albeit in samples that were not 
informative about genetic and environmen -
tal influences (Roberts, Caspi, & Moffitt, 
2003; Roberts & Robins, 2004). The first 
study was based on the Dunedin Longitudi -
nal Study of an entire birth cohort in New 