 9. Behavioral Genetics and Personality  233
about transformations in personality over 
time and across situations. Rather, traits 
that people already possess lead them to 
create experiences and situations that rein -
force and elaborate on those same person -
ality traits. Though the studies to date that 
have provided data supporting this proposi -
tion have not specifically addressed the roles 
of genetic and environmental influences in 
these processes, we can infer them from the 
general knowledge we have that both genetic 
and environmental influences on personal -
ity are pervasive, yet neither is deterministic. 
Future studies should be designed to address 
these roles directly, but we propose that, to 
paraphrase an old expression, genetic in -
fluences on personality affect the beds we 
make, and then we lie in them.
Finding Main Effects of Specific Genes 
on Personality Has Been  Difficult
Underlying the aggregate genetic influ -
ences we have discussed so far are specific 
genomic variants that contribute to popula -
tion variation in personality. Discoveries in 
this area have the potential to revolutionize 
personality research by making it possible to 
use individuals’ specific genotypes to clarify 
causal relations from cells to social process -
es, thereby illuminating how genes influence 
personality development. Actually making 
such discoveries remains challenging, de -
spite many concerted gene searches and im -
provements in techniques used, power to de -
tect specific genetic associations, and great 
reductions in costs of implementing these 
techniques. Molecular genetics personality 
research is described in more detail by de 
Moor (Chapter 10, this volume). Here, we 
briefly review research strategies and the 
general state of the literature in an effort to 
reach some conclusions regarding conceptu -
al challenges inherent in working to connect 
specific polymorphisms and personality.
Two major approaches have been em -
ployed to associate specific genetic variants 
with specific personality variables: candi -
date gene studies and genomewide associa -
tion studies (GWAS). Candidate gene studies 
involve using theory to specify, a priori, the 
genetic variant(s) of interest, whereas GWAS 
involve scanning the entire genome, hunting 
for numerous potentially associated vari -
ants. Variants that are the focus of GWAS are known as single-  nucleotide polymor -
phisms (SNPs). SNPs are located throughout 
the genome and constitute millions of spe -
cific differences from person to person at the 
level of individual nucleotides, the most fun -
damental building blocks of DNA. The SNPs 
that are studied in most GWAS research are 
known as common variants,  because they 
vary notably in the population. Specifically, 
common variants are those SNPs for which 
the less common variant or minor allele  is 
present in at least a notable minority of the 
population (e.g., 1–5%).
Generally speaking, candidate gene stud -
ies preceded GWAS, because the technol -
ogy needed to implement GWAS arrived on 
the scene later than the technology involved 
in genotyping individual variants. As of 
this writing, results from candidate gene– 
personality research have not been convinc -
ingly and consistently replicated (Munafò et 
al., 2003; Munafò & Flint, 2011; Okbay & 
Rietveld, 2015), and the approach has fallen 
out of favor.
There are many specific reasons for limited 
replicability of candidate gene–  personality 
associations, but they can be summarized 
under one general heading: The traits in -
volved are polygenic (Sanchez-  Roige, Gray, 
MacKillop, Chen, & Palmer, 2018). This 
means that many genes, each with small in -
fluence, are involved in expression of each 
trait. We know this to be the case because, 
unlike the wrinkled and not-  wrinkled peas 
studied by Gregor Mendel, the offspring of, 
for example, a stress-  reactive father and a 
calm and unflappable mother are not neatly 
described as either stress reactive or calm 
and unflappable, as they would be if a single 
gene were involved in stress reactivity. Rath -
er, the offspring’s relative stress reactivity 
can be ordered along a basically normally 
distributed continuum. Although most of 
the individual genes involved in personality 
may follow the basic Mendelian patterns of 
transmission and expression, and their ef -
fects may even be neatly additive, no single 
gene is likely to be essential to trait expres -
sion, and possession of any one of them is 
unlikely to determine trait expression. This 
means that two family members who are 
relatively concordant for, say, level of ag -
gression may share relatively few to none of 
the genes relevant to their level of aggression 
with two other family members who are sim -