 1. The Evolution of Human Personality 19
Ellis, 2004; Ellis, Schlomer, Tilley, & Butler, 
2012; Mendle et al., 2009; Neberich, Penke, 
Lehnart, & Asendorpf, 2010; Penke, 2009). 
Whatever the eventual empirical outcomes, 
this example illustrates how selection can 
favor species-  typical adaptations that shunt 
individuals toward different adult strategies, 
depending on exposure to critical cues pres -
ent in developmental environments.
Other human adaptations respond to im -
mediately encountered environmental in -
formation rather than being “locked in” by 
early environmental events. The physiologi -
cal mechanism that results in calluses, for 
example, responds to immediately experi -
enced friction to the skin. Individuals differ 
recurrently in the degree to which they pur -
sue activities that result in frequent repeated 
friction to the skin. Stable individual differ -
ences in calluses, in this example, are proper -
ly understood as functionally patterned dif -
ferences in output stemming from enduring 
environmental differences  in the inputs to 
the callus-  producing mechanism. These en -
during individual differences, like those set 
by early experiential calibration, are the re -
sult of the interaction between environments 
and evolved mechanisms . A similar form of 
functionally patterned individual differences 
can occur with psychological variables.
Enduring Situational  Evocation
Many human adaptations respond to imme -
diately encountered environmental contin -
gencies rather than being “set in plaster” by 
early environmental events. The physiologi -
cal mechanism that results in calluses, for 
example, responds to immediately experi -
enced friction to the skin. Individuals differ 
recurrently in the degree to which they pur -
sue activities that result in frequent repeated 
friction to the skin. The stable individual 
differences in calluses, in this example, are 
properly understood as adaptively patterned 
differences stemming from enduring envi -
ronmental differences in the evocation of the 
callus-  producing mechanism. These endur -
ing individual differences, like those set by 
early experiential calibration, are the result 
of a specific form of interaction between en -
vironments and evolved mechanisms.
A similar form of adaptively patterned in -
dividual differences can occur with psycho -logical factors. If individuals stably occupy 
different environments, evolved psychologi -
cal mechanisms can produce what seem to be 
stable individual differences (Buss & Greil -
ing, 1999). Let us consider a married man 
named Matthew. Assume that Matthew’s 
wife is friendly, likes to dress in reveal -
ing clothing, and enjoys flirting with other 
men. These circumstances can elicit frequent 
“mate poaching” attempts from other men 
over the course of the relationship. These 
events are hypothesized inputs into jealousy- 
producing mechanisms that produce as out -
put suspicions, emotional upset, and mate- 
guarding behavior. By contrast, Matthew’s 
colleague, Michael, possesses the same psy -
chological adaptation of jealousy—  the same 
sexual jealousy mechanism —but is not ex -
posed to any of these cues in his romantic 
relationship and consequently does not ex -
hibit these cognitive, affective, or behavioral 
facets of jealousy. In this example, observers 
might be misled into concluding that Mat -
thew has the “stable trait” of jealousy, and 
that Michael does not. In fact, however, 
Matthew’s recurrent displays of jealousy— 
and the apparent personality differences 
between Matthew and Michael—  are in fact 
attributable to Matthew’s recurrent expo -
sure to jealousy-  activating inputs (Buss & 
Greiling, 1999). In short, stable occupancy 
of different environments can produce stable 
individual differences that appear trait-like, 
even though they are properly explained by 
a species-  typical mechanism that is sensitive 
to environmental inputs.
Individual differences in jealousy, in this 
example, are enduring over time and adap -
tively patterned. They rest on a foundation 
of evolved psychological mechanisms shared 
by all but differentially activated in some. 
Were the enduring environment to change— 
for example, if the man got divorced and 
remarried a woman of equal or lower mate 
value—then the enduring pattern of psy -
chological and behavioral jealousy would 
presumably change (Tooby & Cosmides, 
1990a).
In summary, enduring adaptive individual 
differences results from evocations produced 
by the enduring situations inhabited. These 
relatively enduring situations may include 
one’s overall desirability as a mate on the 
mating market age, and the ratio of men to 