 1. The Evolution of Human Personality 23
variation over time and space can maintain 
both boldness and cautiousness. In short, 
environmental heterogeneity can set up con -
ditions in which heritable individual differ -
ences are maintained in the population. This 
is a form of balancing selection.
Nettle (2006), building on MacDonald 
(1995, 1998, 2005), offered a specific set of 
hypotheses about the adaptive value of being 
high or low on each of the Big Five. For ex -
ample, Nettle characterizes high levels of ex -
traversion as a strategy that fosters success 
in mating, forming social allies, and explor -
ing the environment but also entails costs 
in the form of greater exposure to dangers 
from the physical environment. When envi -
ronments are relatively safe, an extraverted 
strategy may pay off, since the risks incurred 
from the physical environment are minimal. 
In environments that are hazardous, how -
ever, an extraverted strategy may suffer in 
fitness currencies, and instead a more intro -
verted strategy may be favored. The argu -
ment is that temporal variation in the strat -
egy favored by environmental pressures can 
maintain genetic variation on the personal -
ity dimension of extraversion–  introversion.
Nettle argues that each end of the Big Five 
carries benefits as well as costs. Neuroticism 
may give an individual the benefit of being 
extra vigilant about dangers, but it carries 
the cost of increased stress, anxiety, and 
depression. High agreeableness can lead to 
excellent social alliances and valued part -
nerships, but also to vulnerability to being 
cheated or otherwise exploited.
The critical agenda for those pursuing 
this line of theorizing is to identify the spe -
cific costs and benefits of variation in each 
of the key traits across different environ -
ments. Nettle’s (2006) hypotheses provide 
a start, but alternatives could be suggested. 
For extraversion, for example, although one 
benefit may be mating success, if the extra -
verted strategy leads an individual to have 
sex with individuals who are already mated, 
a key cost may be social or physical retalia -
tion from their mates (Buss, 2000). As Penke 
and his colleagues (2007, p. 566) aptly con -
clude, “Even if balancing selection proves to 
be a good general account of heritable per -
sonality traits, much more research would 
be needed to identify each personality trait’s 
relevant fitness costs and benefits across dif -
ferent environments.”Empirical and simulation-  based research 
has provided some evidence that is at least 
consistent with this form of balancing se -
lection. One potential empirical example 
comes from a study that assessed the per -
sonalities of people living in mainland Italy 
and on a number of small islands off the 
coast of Italy (Camperio Ciani, Veronese, 
Capiluppi, & Sartori, 2007). People from 
families that were long-time inhabitants of 
the islands (at least 20 generations) scored 
lower on both extraversion and openness 
to experience compared to those from the 
mainland.  Islanders who descended from 
families that were multigeneration inhabit -
ants of the  islands were also lower on ex -
traversion and openness compared to those 
who were recent immigrants to the islands. 
Although these findings are somewhat cir -
cumstantial, they are consistent with the no -
tion that different environments can select 
for different personality trait levels, and that 
environmental heterogeneity can maintain 
heritable personality variation.
Molecular genetics studies provide further 
evidence for the idea that balancing selection 
can maintain heritability in human person -
ality. Chen, Burton, Greenberger, and Dmi -
trieva (1999) hypothesized that the 7-  repeat 
(7R) allele of the DRD4  gene, which is as -
sociated with novelty seeking (Munafo, Yal -
cin, Willis-  Owen, & Flint, 2008), is favored 
by selection when people migrate to new en -
vironments or inhabit resource-  rich environ -
ments. Chen et al. (1999), provided empiri -
cal support for this hypothesis by studying 
the migration patterns of 2,320 individuals 
from 39 groups and demonstrating that mi -
gratory populations had a far higher pro -
portion of long-  allele DRD4  genes than did 
sedentary populations. These findings could 
potentially be explained by selective mi -
gration of individuals carrying those genes 
rather than selective favoring of those genes 
in the new environments. However, inde -
pendent research has subsequently provided 
evidence that the 7R allele of the DRD4  
gene is more advantageous among nomadic 
than among settled populations (Eisenberg, 
Campbell, Gray, & Sorenson, 2008). Impor -
tantly, this suggests that environmental het -
erogeneity can select for different optimum 
personality trait levels in different environ -
ments. This leads to the maintenance of heri -
table personality variation on that trait.