 1. The Evolution of Human Personality 25
Psychopathic Deviate scale (Willerman, 
Loehlin, & Horn, 1992). Second, some psy -
chopaths appear to pursue an exploitative 
short-term sexual strategy, which could be 
the primary route through which genetic 
variants associated with psychopathy in -
crease or are maintained in the population 
(Rowe, 1995). Psychopathic men tend to be 
more sexually precocious, to have sex with 
a larger number of women, to have more 
children out of wedlock, and are more likely 
to separate from their wives (Rowe, 1995). 
This short-term, opportunistic, exploitative 
sexual strategy would be expected to rise 
in populations marked by high mobility, in 
which the reputational costs associated with 
such a strategy would be least likely to be 
incurred (Wilson, 1995).
There are several challenges to this 
theory, such as whether psychopathy rep -
resents a type or a continuum (Baldwin, 
1995; Eysenck, 1995), whether its frequen -
cy is sufficiently large to be maintained by 
frequency-  dependent selection, and whether 
it represents a recently evolved cluster in 
modern populations or an ancient evolved 
strategy (Wilson, 1995; but see Mealey’s 
(1995) response to these challenges). Despite 
these complications, Mealey’s theory of psy -
chopathy nicely illustrate the possibility that 
heritable alternative strategies can be main -
tained by frequency-  dependent selection.
Nonselective  Models: Selective  Neutrality 
and Mutation‑Selection  Balance
A prevalent misconception, perhaps due to 
the term evolution  being used interchange -
ably with selection,  which is just one of 
multiple evolutionary processes, is that 
evolution only produces functional adapta -
tions. In reality, evolution consists of both 
selective and nonselective processes. Here, 
we discuss several nonselective evolutionary 
models in the context of personality psy -
chology.
Outside of the special case of balancing 
selection, selection generally reduces genetic 
variation by purging less successful genetic 
variants from a population. However, this is 
an iterative process across generations that is 
inexorably accompanied by an influx of mu -
tations that introduce new genetic variation. 
Mutations can occur on any of the approxi -mately 20,000 genes that characterize the 
human genome or in any of the regulatory 
parts of the genome. Recent molecular ge -
netics evidence suggests that approximately 
70 new mutations are introduced per indi -
vidual per generation (Keightley, 2012). This 
is how genetic variation is introduced  into a 
population.
Although heritable variants that have 
greater replicative success continue to in -
crease in relative frequency in the popula -
tion, displacing less successful genetic vari -
ants, there are constraints on selection’s 
ability to winnow out these less successful 
genetic variants. The constraints include 
selective neutrality  and mutation–  selection 
balance .
Selective  Neutrality
Some mutations neither disrupt nor improve 
the functioning of an organism’s brain or 
other organs. Because these mutations have 
no impact on an organism’s reproductive 
success, selective processes will neither in -
crease nor decrease their relative frequency 
in the population. Such mutations are selec -
tively neutral .
Early evolutionarily informed work on 
personality identified selective neutrality as 
one possible explanation for the heritability 
of personality differences (Tooby & Cos -
mides, 1990a). However, selective neutral -
ity carries a highly implausible assumption: 
that different personality trait levels are un -
linked to reproductive fitness. This is clearly 
at odds with known predictive relation -
ships between personality and life outcomes 
(Nettle, 2006; Ozer & Benet-  Martinez, 
2006; Roberts, Kuncel, Shiner, Caspi, & 
Goldberg, 2007). Many personality traits, 
including those captured by the five-  factor 
model, are systematically linked with com -
ponents that contribute to fitness (Buss & 
Greiling, 1999; Nettle, 2006; Ozer & Benet- 
Martinez, 2006; Roberts et al., 2007). Con -
scientiousness, for example, is linked with 
literal survival in the form of longevity 
(Deary, Weiss, & Batty, 2010). Extraversion 
is linked with sexual access to number of 
sex partners (Nettle, 2005; Schmitt, 2004). 
The key point is that many personality traits 
have been linked to components tributary 
to reproductive success, as well as to over -