24 I. THEORETICAL  PERSPECTI vES AND  CONCEPT uAL uNITS
Recent simulation-  based work also shows 
how heterogeneity in the social environ -
ment can maintain personality differences. 
Del Giudice (2012) showed that sex ratios 
of populations will show substantial natu -
ral fluctuation over time. Since different sex 
ratios greatly alter the cost and benefits of 
different reproductive strategies in humans 
(Schmitt, 2004), fluctuation in sex ratios 
can exert balancing selection pressure on 
reproductive strategy-  related personality 
dimensions such as sociosexuality (Schmitt, 
2004), as well as extraversion and neuroti -
cism (Alvergne, Jokela, & Lummaa, 2010).
Heritable Frequency‑Dependent 
Adaptive  Strategies
Strictly speaking, frequency-  dependent  se-
lection  is not a second “type” of balanc -
ing selection; rather, it is a particular case 
of balancing selection due to environmental 
heterogeneity. For conceptual clarity, we 
nonetheless find it useful to give it its own 
treatment.
Frequency-  dependent selection occurs 
when multiple strategies exist in a popu -
lation, and the fitness of each strategy de -
creases as that strategy’s frequency increases 
relative to other strategies in the population. 
Biological sex is the most obvious example 
of frequency-  dependent selection. As the 
ratio of males-to-  females in a population 
increases, the average fitness of males de -
creases, and vice versa; that is, the familiar 
example of an approximately equal ratio 
of women to men is a result of frequency- 
dependent selection .
Alternative adaptive strategies can also 
be maintained within sex by frequency- 
dependent selection. Among the bluegill 
sunfish, for example, three different male 
mating strategies are observed: a “parental” 
strategy that defends the nest, a “sneak” 
strategy that matures to only a small body 
size, and a “mimic” strategy that resembles 
the female form (Gross, 1982). The sneak -
ers gain sexual access to the female eggs by 
avoiding detection due to their small size, 
while the mimics gain access by resembling 
females, thus avoiding aggression from the 
parental males. As the parasitizing strate -
gists increase in frequency, however, their 
success decreases—  their existence depends on the parentals, who become rarer as the 
parasites become more common, render -
ing the parasite strategies more difficult to 
pursue. Thus, heritable alternative strategies 
within sex are maintained by the process 
of frequency-  dependent selection. Theoreti -
cally, these heritable individual differences 
can persist in the population indefinitely 
through frequency-  dependent selection, 
unlike the process of directional selection, 
which tends to drive out heritable variation.
Psychopathy as a Frequency-  Dependent 
Strategy.  Frequency-  dependent selection has 
also been hypothesized to explain the per -
sonality constellation that characterizes psy -
chopathy (see Mealey, 1995). Psychopathy  
(sometimes called sociopathy  or antisocial 
personality disorder ) represents a cluster 
of traits marked by egotism, unreliable be -
havior, impulsivity, an absence of enduring 
relationships, superficial social charm, and 
low levels of social emotions such as guilt, 
love, and empathy (Cleckley, 1982; Ameri -
can Psychiatric Association, 1994).
Psychopaths pursue a social strategy 
characterized by exploiting the reciprocity 
mechanisms of others. After feigning coop -
eration, psychopaths typically defect.
According to the theory, a psychopathic 
strategy can be maintained by frequency- 
dependent selection. As the number of cheat -
ers increases, and hence the average cost to 
the cooperative hosts increases, mechanisms 
would presumably evolve to detect cheat -
ing and to inflict costs on those pursuing 
a cheating strategy. As the prevalence of 
psychopaths increases, therefore, the aver -
age payoff of the psychopath’s strategy de -
creases. However, because over 95% of both 
men and women do not  exhibit psychopa -
thy, the “cheating” strategy of psychopathy 
has a large pool of resources to exploit: the 
majority strategy of cooperation. As long as 
the frequency of psychopaths is not too high, 
psychopathy can be maintained amid a pop -
ulation composed primarily of cooperators 
(Mealey, 1995).
There is some indirect evidence that is 
consistent with Mealey’s hypothesis. First, 
behavioral genetics studies suggest that psy -
chopathy may be moderately heritable, at 
least as indicated by the Minnesota Mul -
tiphasic Personality Inventory (MMPI) 