236 II. BIOLOGICAL  FOuNDATIONS
any specific genes will necessarily lead to a 
specific level on a personality trait, and we 
cannot expect that any specific genes will 
only influence one particular trait. Instead, 
the influences of specific genes are likely to 
be contingent on other factors such as en -
vironmental circumstances or presence of 
other genes. Finally, the proteins that are the 
direct products of gene expression are only 
initial steps in long series of transactional 
processes that lead to a particular personal -
ity phenotype, and these processes may be 
interrupted or altered at many stages and 
thus lead to a wide range of phenotypes.
Everyone has a personality and manifests 
behaviors shared with all other humans. Ev -
eryone is aggressive in some situations and 
passive in others. What makes each individ -
ual personality unique is the distinct eliciting 
circumstances, and the unique combination 
of underlying behavioral systems relevant to 
personality (e.g., individual differences in 
sensitivity to specific eliciting circumstances, 
intensity of resulting behaviors, and so on). 
Consider, as an analogy, physical health. 
Everyone has cardiovascular, respiratory, 
and digestive systems that perform the same 
basic functions with varying degrees of effi -
ciency. Health lies in optimal functioning of 
these systems, and illness takes idiosyncratic 
forms depending on eliciting circumstances 
and particular constellations of genetic vul -
nerabilities.
For physical health as measured by num -
bers of chronic illnesses, we (Johnson & 
Krueger, 2005) observed that genetic vari -
ance is greater with lower monetary in -
come and lower perceived control over life. 
In other words, under psychologically and 
physically stressful environmental condi -
tions, genetic vulnerabilities to illness were 
more likely to be expressed—  a form of G 
× E interaction. These results also indicat -
ed patterns of rGE, which further influence 
health outcomes, beyond G × E interactions. 
Importantly, the expressed genetic vulner -
abilities took many different forms involv -
ing different physiological systems; without 
doubt they involved many different genes. 
The situation for personality may ultimately 
prove to be similar, such that correlational 
and interactive personality processes can 
be understood by modeling rGE and G × E, 
without necessarily identifying the specific genetic polymorphisms involved in these ef -
fects (South, Hamdi, & Krueger, 2017).
What Can We Learn from Understanding 
the Link between  Quantitative and  Molecular 
Genetics  Research?
If everything is heritable, why is it so hard to 
find the genes involved in personality? The 
key to unraveling this riddle may lie in under -
standing just how much variation is glossed 
over by a general heritability statistic. As 
one example, we (Krueger et al., 2008) ex -
amined how genetic and environmental con -
tributions to personality traits might change 
as a function of the nature of adolescents’ 
relationships with their parents. Across a 
range of parental regard (e.g., “I admire my 
parent and my parent admires me”), genetic 
contributions to positive emotionality (akin 
to extraversion) varied widely, from 34% at 
low levels of regard to 77% at high levels 
of regard. Fifty percent is a good “halfway 
point” (indeed, the heritability of positive 
emotionality was 52% when we estimated 
it without modeling regard as a moderator). 
But this very general 52% value disguised a 
wide range of etiological scenarios, in which 
genetic effects on positive emotionality were 
markedly diminished and enhanced. In sum -
mary, we might ask not “how heritable a trait 
is” but rather “what are the circumstances 
in which genetic contributions to this trait 
are enhanced or suppressed?”—and look for 
specific polymorphism–  personality associa -
tions in circumstances in which genetic con -
tributions are enhanced.
ACKNOWLEDGMENTS
We acknowledge past support for our collabora -
tive work from National Institute on Aging Grant 
No. AG20166. Robert Krueger’s efforts were 
supported by the National Institutes of Health 
Grant Nos. R01AG053217 and U19AG051426, 
and by the Templeton Foundation.
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