 1. The Evolution of Human Personality 27
ity being under mutation–  selection balance, 
an interpretation that we discuss in the fol -
lowing section.
A Comprehensive  Framework: 
Integrating  Selective and  Nonselective  Models
We have presented five evolutionary models 
for the origins of variation in human person -
ality. These models are based on both selec -
tive and nonselective processes, and each 
offers a potential account for heritable indi -
vidual differences. A central impetus for pre -
senting these models together in a single re -
source is that evolutionarily informed work 
to date has generally emphasized just one or 
a few of these models (e.g., Lewis, 2015). 
Such a narrow focus has enabled research -
ers to offer useful expansions on individual 
models. However, their restricted scope has 
led to not only the exclusion of other models 
but also implicit and unnecessary antago -
nism between models.
This is problematic for two reasons. First, 
the models are not necessarily mutually ex -
clusive. Second, there is no sound reason to 
believe that just one model should account 
for all heritable variation in personality. 
Human personality exhibits heritable vari -
ance on manifold dimensions. Different evo -
lutionary models may account for variance 
on different dimensions. Moreover, different 
models may account for distinct proportions 
of variance within each dimension. A central 
objective for personality researchers should 
therefore be to integrate these distinct evolu -
tionary models to maximize the explanatory 
power of their research framework.
For researchers interested in identify -
ing the relative explanatory contributions 
offered by each of the evolutionary mod -
els presented here, it is important to use 
evidentiary criteria capable of discriminat -
ing between these distinct models. This is 
key, because many criteria are consistent 
with multiple evolutionary models and are 
therefore not capable of discriminating be -
tween alternative evolutionary models. For 
example, Camperio Ciani and colleagues’ 
(2007) finding that Italian islanders differ 
on major personality axes relative to Italian 
mainlanders is consistent with balancing se -
lection favoring different optimal personal -
ity levels in different environments but also with universal psychological mechanisms 
responding to different environmental input.
Similarly, mutation–  selection balance and 
condition-  dependent psychological adap -
tations lead to identical  expected patterns 
of genetic variation; that is, a condition-  
dependent model of personality proposes 
that if psychological mechanisms calibrate 
manifest personality trait levels according 
to condition-  linked inputs such as physical 
attractiveness and strength, and these attri -
butes themselves are heritable, then patterns 
of heritable variation in personality may ac -
tually reflect heritable variation in these in -
puts, not in the psychological mechanisms 
themselves. Verweij and colleagues’ (2012) 
are not unjustified in concluding that the 
genetic variation associated with personal -
ity differences in their data is consistent with 
mutation–  selection balance. However, they 
jump to the conclusion that the genetic loci 
under mutation–  selection balance are spe -
cifically those that code for the mechanisms  
that produce personality. A plausible alterna -
tive is that the genetic loci under mutation–  
selection balance are those that code for the 
physical features that serve as inputs  to those 
personality-  regulating mechanisms. These 
ideas illustrate that a sophisticated evolution -
ary approach to personality requires an inte -
grated understanding of multiple selective as 
well as nonselective evolutionary processes.
Adaptations for Detecting and Acting 
on Personality  Differences
Whether individual differences are adaptive -
ly patterned or not, other individuals consti -
tute one of the primary environments within 
which humans function (Alexander, 1987) 
and define a major part of the human adap -
tive landscape (Buss, 1991). Other individu -
als can both pose  and help solve adaptive 
problems. Attending to personality differ -
ences in others can be crucial for solving (or 
avoiding) adaptive problems. For example, 
assessing whether others are pursuing co -
operative or defecting social strategies can 
result in mutually beneficial reciprocal alli -
ances on the one hand, or disastrous con -
sequences on the other, such as resources 
pilfered, reputations damaged, and pregnan -
cies unwanted.