 1. The Evolution of Human Personality 29
Personality theories would be incomplete 
without an account of the major ways in 
which individuals differ. Although some 
individual differences may be random, and 
hence independent of the basic functioning 
of human nature, the most important indi -
vidual differences are those that stem from 
the workings and nature of the species- 
typical mechanisms.
Major individual differences can originate, 
in principle, from environmental sources of 
variation, genetic sources of variation, or a 
combination of the two. It may seem ironic 
to some that the environmentally based in -
dividual differences are most easily han -
dled within this evolutionary psychological 
framework. Individual differences can result 
from early experiences during childhood, 
enduring environmental experiences, strate -
gic specialization, and other forms of vary -
ing environmental input into species-  typical 
mechanisms.
All individuals possess callus-  producing 
mechanisms but differ in the thickness and 
distribution of calluses because of individual 
differences in experiences of friction to the 
skin. Similarly, all individuals may possess 
a psychological mechanism of jealousy but 
differ in the degree to which they enduringly 
occupy an environment filled with threats to 
their romantic relationship, perhaps because 
they are mated with someone who is habitu -
ally flirtatious or who is higher in “mate 
value,” and so gives off signals of defection 
(Tooby & Cosmides, 1990b). Stable individ -
ual differences in jealousy, in this example, 
arise from stable individual differences in 
inhabiting jealousy-  evoking environments.
Balancing selection, where traits can be 
maintained as a consequence of differences 
in selective environments over time or space, 
is an evolutionary process that can poten -
tially explain personality characteristics that 
are heritable. Negative frequency-  dependent 
selection is a specific form of balancing se -
lection, an evolutionary process that can 
create heritable individual differences that 
are adaptively patterned.
Not all individual differences, of course, 
will be adaptively patterned. Some might 
occur because of random environmental 
forces, genetic noise, genetic defects, and 
other factors. Some individual differenc -
es, such as those in intelligence or mental health, may reflect differences in mutation 
load (Keller & Miller, 2006).
Given this early stage in the development 
of evolutionary personality psychology, no 
pretense is made that we have arrived at, or 
even approximated, the ultimate theory of 
personality. In this chapter we have merely 
offered some suggestions for ingredients 
that might be contained within a future 
theory of personality. Evolutionary psychol -
ogy, however, does offer something lacking 
in all existing nonevolutionary theories of 
personality—  a nonarbitrary anchoring for 
the specification of the basic psychological 
machinery that humans share, as well as the 
major ways in which humans differ.
ACKNOWLEDGMENTS
This chapter is an updated and expanded version 
of the earlier chapter by D. M. Buss in the third 
edition of this handbook. Some sections include 
material also discussed in Buss and Penke (2015) 
in Lewis, Al-  Shawaf, and Buss (in press), and 
in Lewis, Al-  Shawaf, Thompson, and Buss (in 
press).
NOTE
1. Contrary to views that perpetuate the 
false dichotomies of nature–  nurture or genetic– 
environmental, evolved psychological mechanisms 
are necessarily entailed by theories of environ -
mental influence (Tooby & Cosmides, 1990a) and  
environmental influence is necessarily entailed by 
evolved psychological mechanisms; without receiv -
ing the environmental input activating it, an evolved 
psychological mechanism does not produce its out -
put.
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