22 I. THEORETICAL  PERSPECTI vES AND  CONCEPT uAL uNITS
differences in the psychological mechanisms  
responsible for personality, or (2) differenc -
es in characteristics that serve as input  into 
personality mechanisms and thereby influ -
ence their output. For example, the number 
of nucleotide repeats at the AR locus is asso -
ciated with individual differences in attrac -
tiveness and strength, which predict indi -
vidual differences in extraversion above and 
beyond genetic polymorphism (Lukaszewski 
& Roney, 2011).
This relationship between extraversion, 
physical formidability, and attractiveness 
offers a clear empirical illustration of how 
species-  typical evolved psychological mech -
anisms could produce heritable personality 
outcomes. If the costs and benefits of ex -
traversion vary as a function of individuals’ 
heritable attributes such as physical attrac -
tiveness and strength, and extraversion- 
producing mechanisms regulate their outputs 
based on these attributes, then we should ex -
pect extraversion to show up as heritable in 
behavior genetics studies—  but only because 
it is dependent on physical attractiveness and 
strength, which are themselves heritable (see 
Tooby & Cosmides, 1990a, for their semi -
nal discussion of this concept, which they 
termed “reactive heritability”).
In this case, heritable individual differ -
ences in physical formidability and attrac -
tiveness provide input into evolved psycho -
logical mechanisms that thereby produce 
individual differences in extraversion levels 
as output. In this example, the extraver -
sion is not directly heritable, but rather is 
“reactively heritable” in the sense that it is 
a secondary consequence of heritable physi -
cal features that provide input into species- 
typical mechanisms.
More generally, if the psychological mech -
anisms that produce personality take as 
input cues ancestrally linked to the probabi -
listic costs and benefits of alternative person -
ality trait levels, and those costs and benefits 
vary as a function of individuals’ heritable 
attributes (e.g., strength and physical attrac -
tiveness), then we should expect personality 
to show heritability in behavioral genetics 
studies as the output of species-  typical psy -
chological mechanisms.
Although species-  typical psychological 
mechanisms can, in this way, produce indi -
vidual differences that are consistent with 
personality’s well-known and established stability and heritability, there are other 
evolutionary models for the heritability of 
personality. These include other selective 
models (e.g., balancing selection rather than 
directional selection), as well as nonselective 
evolutionary models.
Balancing  Selection
In general, the process of directional se -
lection tends to use up heritable variation. 
Heritable variants that are more successful 
tend to replace those that are less success -
ful, resulting in species-  typical adaptations 
that show little or no heritable variation in 
the presence or absence of basic functional 
components (Williams, 1966).
In balancing selection, genetic varia -
tion (originally introduced by mutation) is 
not weeded out by selection, but rather is 
maintained by selection. Balancing selec -
tion occurs when different environmental 
conditions favor different levels of a trait 
dimension; heritable variation can be main -
tained. There are several forms of balancing 
selection, but the two most relevant forms 
for personality appear to be environmental 
heterogeneity of selection pressures over 
time and space and frequency-  dependent 
selection  (Penke et al., 2007).
Environmental Heterogeneity of  Selection 
Pressures over Time and Space
If environments vary, then selection can 
favor not just a single optimal value for a 
trait, but rather different optima in the dif -
ferent environmental conditions. Consider 
as an example variations over time or place 
in the scarcity or abundance of food resourc -
es. Under conditions of food scarcity, selec -
tion might favor boldness and risk taking 
that impel an individual to venture out into 
unknown territory to find food. Under such 
circumstances, timidity and risk aversion 
may lead to starvation and death. By con -
trast, under conditions of food abundance, 
bold and risky exploratory behavior could 
needlessly expose an individual to preda -
tors; aggressive, unfamiliar humans; and 
other biotic and abiotic ecological threats. 
Under these conditions, selection would 
favor greater levels of timidity. If some en -
vironments favor boldness and others favor 
cautiousness, as in this example, then this 