18 I. THEORETICAL  PERSPECTI vES AND  CONCEPT uAL uNITS
ful heuristic for generating and testing novel 
hypotheses about individual differences. 
Because psychological mechanisms are acti -
vated by cues ancestrally linked to the par -
ticular problem they were designed to solve, 
they will be activated only in particular 
contexts—  to which only some humans are 
exposed. Crucially, this suggests that human 
nature will express itself not as invariant 
impulses, as implied by some personality 
theories, but rather in variable and context- 
dependent ways. Here, we outline several 
distinct ways in which evolved psychological 
mechanisms can be activated differentially 
across individuals as a function of different 
environmental inputs, thereby resulting in 
individual differences in personality.
Early Experiential  Calibration
Humans have some adaptations that are 
designed to produce stable individual differ -
ences due to exposure to events during criti -
cal periods of development. Individuals who 
share common underlying psychological 
mechanisms can experience different envi -
ronmental events early in their development 
that serve as inputs into those mechanisms 
and thereby channel those individuals into 
alternative, developmentally stable strategies 
characterized by distinct psychological and 
behavioral outputs.
Belsky, Steinberg, and Draper (1991), 
for example, propose that early exposure 
to stressful environments, such as resource 
scarcity, parental unpredictability, or father 
absence serve as key inputs into evolved 
psychological mechanisms that produce, as 
output, developmentally stable differences 
in mating strategy. According to Belsky 
and colleagues, the presence of a father in 
the home during the first 5–7 years of life is 
a reliable cue that, in the individual’s local 
environment, adult pair-bonds are enduring 
and levels of paternal investment are high. In 
such an environment, the benefits of pursu -
ing a committed, long-term mating strategy 
are greater, on average, than a mating strat -
egy characterized by uncommitted sexual 
relations and frequent partner switching. 
Accordingly, selection would have shaped 
psychological mechanisms to take this envi -
ronmental cue of father presence as input, 
and to functionally calibrate individuals’ mating strategy according to this cue. Indi -
viduals who are exposed to the cue of father 
presence exhibit a suite of psychological and 
behavioral features consistent with this the -
ory: delayed sexual maturation, later onset 
of sexual activity, a search for long-term 
securely attached adult relationships, per -
ceptions of others as reliable and trustwor -
thy, expectations that relationships will be 
enduring, and heavy investment in a small 
number of children. Conversely, individu -
als who are exposed to the cue of father ab -
sence exhibit earlier sexual maturation, ear -
lier sexual initiation, and frequent partner 
switching; perceive others as untrustworthy 
and relationships as transitory; and oppor -
tunistically attain and extract resources 
from uncommitted sexual liaisons.
Although Belsky and colleagues (1991) 
did not explicitly advance these ideas using 
the concept of evolved psychological mech -
anisms, all theories of environmental in -
fluence  ultimately rest on a foundation of 
evolved psychological mechanisms, wheth -
er they are acknowledged as such or not 
(Tooby & Cosmides, 1990a).1 In the case of 
Belsky and colleagues, the implicit psycho -
logical mechanisms are specifically designed 
to take as input information about the pres -
ence and reliability of paternal investment, 
and process that input to produce, as output, 
distinct mating strategies.
Crucially, the individual differences that 
result from early experiential calibration 
are functionally calibrated to local envi -
ronmental cues.  This suggests that these 
environment-  contingent strategies resulted 
from a long and recurrent evolutionary his -
tory in which different individuals confront -
ed radically different rearing environments. 
Environmental variation over human evolu -
tionary history presumably selected for de -
velopmentally flexible mechanisms that take 
as input cues from the rearing environment 
that are predictive of the adult environment 
the individual is likely to encounter.
Although some variants of this hypothesis 
have received mixed empirical support at 
best (e.g., effects of childhood father absence 
on adult female mating strategies), other 
versions appear more empirically plausible 
(e.g., effects of childhood stress on acceler -
ated female puberty and perhaps on roman -
tic attachment security; Del Giudice, 2009; 