234 II. BIOLOGICAL  FOuNDATIONS
ilarly concordant and similarly aggressive 
but from another family. This may be more 
the case for more broadly defined traits. To 
complicate the picture, most of the genes 
involved in commonly occurring personal -
ity traits are likely to be pleiotropic —which 
means that these genes have more than one 
function: They are likely involved in many 
different biochemical processes throughout 
the brain and thus may be associated with 
several different personality traits, no mat -
ter how independent those traits are mani -
fested behaviorally.
Consistent with the theoretical conceptu -
alization of personality as highly polygenic, 
GWAS research on individual differences 
suggests that numerous genetic variants are 
associated with human individual differ -
ences of all kinds. Within the broad GWAS 
rubric, two approaches to initial analysis of 
genomewide data have been implemented. 
The first approach examines the aggregate 
variation in SNPs across the genome, and 
associates this variation with variation in a 
personality variable. This approach is con -
ceptually akin to twin and adoption designs, 
but instead of using known relationships 
among family members to identify genetic 
variance, it uses measured genomewide 
markers (SNPs) to identify genetic variance 
among unrelated persons.
One popular software package for this 
“molecular variance decomposition ap -
proach” is known as genomewide complex 
trait analysis (GCTA). As a result, this ap -
proach is frequently known as GCTA (not 
coincidentally, these are also the labels for 
the four molecular bases in DNA). GCTA 
can estimate the total “genomic heritability” 
of personality (i.e., the total variation in per -
sonality that can be attributed to aggregate 
variation in SNPs across the genome) but is 
not designed to single out specific SNPs.
Generally speaking, GCTA suggests that 
the heritability of various personality traits 
is greater than zero but smaller than the es -
timates from twin research (e.g., Power & 
Pluess, 2015; Realo et al., 2017; Rietveld et 
al., 2013; Vinkhuyzen et al., 2012). There 
may be a number of reasons for this discrep -
ancy, but one potential reason is that GCTA 
estimates only sum genetic variation derived 
from the study of common variants. As de -
scribed earlier with reference to twin studies, MZ twins share the combinations of com -
mon and uncommon alleles (forms of genes), 
and if such combinations are important to 
personality variation, this genomewide non -
additive variation is reflected in similarity of 
MZ twins, but not by the GCTA approach. 
This implies that the yield of GWAS research 
focused on identifying specific SNPs may be 
modest relative to the heritability estimates 
derived from twin research.
Results from large personality-  relevant 
GWAS consortia are just beginning to ap -
pear in the literature. Recent consortia stud -
ies focused on neuroticism and related traits 
have convincingly identified a handful of 
specific associated SNPs, albeit these SNPs 
account for only a very small amount of pop -
ulation variation in neuroticism (de Moor et 
al., 2015; Okbay, Beauchamp, et al., 2016). 
More promising results are found for educa -
tional attainment, with a recent publication 
reporting 74 associated loci (Okbay, Basel -
mans, et al., 2016). Although educational 
attainment (EA) may seem distal from some 
aspects of personality (e.g., those beyond 
cognitive ability), twin research suggests 
genetically mediated associations linking 
traits in the openness and conscientious -
ness domains with EA (Tucker-  Drob, Briley, 
Engelhardt, Mann, & Harden, 2016). This 
suggests the possibility of working with 
EA- relevant SNPs to predict personality- 
relevant qualities such as self-  control and 
academic motivation (Belsky et al., 2016; 
Smith-  Woolley, Selzam, & Plomin, 2019). In 
summary, as of this writing, the GWAS con -
sortium approach is beginning to yield SNPs 
that are credibly associated with personal -
ity, albeit the numbers of associated SNPs 
are small, and they account for only small 
amounts of personality variance.
Contemplating replication problems in 
candidate gene research, and the modest 
yield of GWAS to date, yields a number of 
conclusions regarding challenges that are 
inherent in seeking potential molecular cor -
relates of personality. As we see it, the basic 
problem is that everyone has a unique per -
sonality, yet everyone also expresses all the 
basic personality traits to some degree in di -
verse situations. Moreover, there are likely 
many different pathways within the brain 
to actualize any behavior related to person -
ality. This multiplicity may be the reason 