 1. The Evolution of Human Personality 21
psychological mechanisms that regulate 
their output according to cues from the in -
dividual’s phenotype are called condition-  
dependent  psychological adaptations.
Recent evolutionarily informed research 
on extraversion, as well as aggression and 
anger, provides empirical support for the 
existence of such condition-  dependent ad -
aptations. High levels of extraversion can 
lead to increased mating opportunities both 
by engaging potential mates and by leading 
to the formation of friendships and valu -
able social alliances (Nettle, 2005, 2006; 
Denissen & Penke, 2008). Because more 
attractive individuals are more likely to 
reap mating benefits from the social inter -
actions that extraverted behaviors create, 
we should expect selection to have favored 
extraversion-  regulating mechanisms that 
are sensitive to individuals’ own physical 
attractiveness. Indeed, research shows that 
physical attractiveness predicts extraver -
sion among both men and women (Lukas -
zewski & Roney, 2011). However, because 
mates and high social status are contested 
resources, extraverted behavior can also 
expose individuals to competition and con -
flict. Evidence ranging from sex differences 
in upper body strength to greater fighting 
success and higher status among stronger 
men in modern-  day hunter–  horticulturalist 
societies suggests that, ancestrally, conflict 
over these resources ancestrally was more 
prevalent among men than among women 
(Lukaszewski & Roney, 2011). Consistent 
with this, physical strength predicts extra -
version among men but not women.
A similar line of reasoning and set of ob -
servations has been made with anger, an -
other dimension on which individuals ex -
hibit stable individual differences. Because 
a mesomorphic man can more successfully 
pursue an aggressive strategy of resource 
acquisition than can an ectomorphic man 
(Sell, Tooby, & Cosmides, 2009; Price, 
Dunn, Hopkins, & Kang, 2012), selection 
should have shaped the mechanisms respon -
sible for producing aggression to be sensitive 
to (i.e., take as input) the individual’s own 
body type as a cue to the ability to success -
fully achieve goals via an aggressive strategy 
(modified from Tooby & Cosmides, 1990a, 
p. 58). Research on anger (Sell et al., 2009) 
has empirically substantiated this idea. In -dividual differences in anger thresholds are 
predictable a priori on the basis of one’s abil -
ity to inflict costs on others; stronger men 
tend to anger more easily and more readily 
(Sell et al., 2009).
The Heritability of Personality Differ -
ences.  An important consideration to take 
from a condition-  dependent model of per -
sonality is that many of the characteristics 
constituting an individual’s morphological 
phenotype, including physical attractiveness 
and strength, are highly heritable (Gao et 
al., 2006; Rowe, Clapp, & Wallis, 1987; Sil -
ventoinen, Magnusson, Tynelius, Kaprio, & 
Rasmussen, 2008; Thornhill & Gangestad, 
1994). This is important, because the notion 
of species-  typical personality mechanisms 
seems, on the surface, to be inconsistent with 
the heritability of personality traits (Penke et 
al., 2007). Indeed, the heritability observed 
in personality traits is not the typical pattern 
expected of species-  typical adaptations.
A number of scholars have interpreted 
the heritability of manifest personality as 
evidence for variation in the genes coding 
for the mechanisms  responsible for produc -
ing personality (Nettle, 2006; Penke et al., 
2007). Some evidence is consistent with 
this view. There are several known genetic 
polymorphisms associated with individual 
differences in personality. For example, neu -
roticism is associated with the short allele 
of the dopamine D4 ( DRD4 ) exon III poly -
morphism (Tochigi et al., 2006); the num -
ber of cytosine–  adenine–  guanine (CAG) re -
peats at the androgen receptor (AR) gene in 
men (Westberg et al., 2009); the serotonin 
transporter promoter 5-HTTLPR short al -
lele (Sen, Burmeister, & Ghosh, 2004); and 
the gamma-  aminobutyric acid [GABA(A)] 
Alpha 6 receptor Pro385Ser Pro allele (Sen 
et al., 2004). The number of CAG repeats at 
the AR locus is also associated with individ -
uals’ extraversion (Lukaszewski & Roney, 
2011; Westberg et al., 2009) and sexual jeal -
ousy levels (Lewis, Al-  Shawaf, Janiak, & 
Akunebu, 2018).
However, models of personality based on 
genetic polymorphisms leave the lion’s share 
of interindividual variation in personality 
unexplained (e.g., see Westberg et al., 2009). 
Furthermore, it is unclear whether these ge -
netic polymorphisms are associated with (1) 