20 I. THEORETICAL  PERSPECTI vES AND  CONCEPT uAL uNITS
women in the local population (Pedersen, 
1991). Future research could profitably ex -
plore these and other features of enduring 
environments as sources of relatively stable 
individual differences.
Strategic  Specialization
From an evolutionary perspective, compe -
tition is keenest among those pursuing the 
same strategy. As one niche becomes more 
and more crowded with competitors, one’s 
success can suffer compared with those 
seeking alternative niches (Maynard Smith, 
1982; Wilson, 1994). Selection can favor 
mechanisms that cause some individuals to 
seek niches where the competition is less in -
tense, and hence where the average payoff 
may be higher.
Mating provides some clear examples. If 
most women pursue the man with the high -
est status or greatest resources, then some 
women would achieve more success by 
courting males outside of the arenas in which 
competition is keenest. In a mating system 
in which both polygyny and monogamy are 
possible, for example, a woman might be 
better off securing all of the resources of a 
lower-  status monogamous man rather than 
having to settle for a fraction of the resourc -
es of a high-  status polygynous man.
The ability to exploit a niche will depend 
on the resources and personal character -
istics an individual brings to the situation, 
whether environmental or heritable in origin 
(Buss, 1989a; Gangestad & Simpson, 2000). 
One variable that is not  heritable is birth 
order. It is possible that firstborns and sec -
ondborns have faced, on average, recurrent -
ly different adaptive problems over human 
evolutionary history. Sulloway (1996), for 
example, argues that firstborns occupy a 
niche characterized by strong identification 
with parents and other existing authority 
figures. Second borns, in contrast, have less 
to gain by authority identification and more 
to gain by overthrowing the existing order. 
According to Sulloway, birth order influenc -
es niche specialization. Second borns devel -
op a different personality marked by greater 
rebelliousness, lower levels of conscientious -
ness, and higher levels of openness to new 
experiences. Birth-order differences show 
up strongly among scientists, where second borns tend to be strong advocates of scientif -
ic revolutions; firstborns tend to strenuously 
resist such revolutions (Sulloway, 1996).
Whether or not the details of Sulloway’s 
arguments turn out to be correct, the ex -
ample illustrates strategic niche specializa -
tion. Individual differences are adaptively 
patterned, but they are not  based on heri -
table individual differences. Rather, birth 
order, a nonheritable individual difference, 
provides input (presumably through interac -
tions with family members) into a species- 
typical mechanism that canalizes strategic 
niche specialization.
All of these individual differences are ex -
amples of environmentally contingent in -
dividual differences. In early experiential 
calibration,  the developmental environment 
activates stable individual differences in 
adulthood. In enduring situational evoca -
tion, the stable occupancy of different en -
vironments activates and maintains stable 
individual differences in personality. And 
in strategic specialization,  the existence or 
lack of existence of environmental or social 
niches to exploit activates and maintains 
stable individual differences. We now turn 
to those individual differences that have at 
least a partial basis in heritable individual 
differences.
Adaptive Self‑Assessment of  Heritable  Qualities
Any feature of the environment that influ -
ences the successful attainment of fitness- 
relevant outcomes may serve as an input 
into evolved psychological mechanisms 
(Tooby & Cosmides, 1990a, p. 59). These 
different forms of environmental input can 
range from immediate situational inputs to 
stable differences in an individual’s pheno -
type (Penke, 2011). This may even include 
features of an individual’s own body. To the 
extent that fitness-  relevant outcomes were 
ancestrally linked to an individual’s own 
physical features, selection should have fa -
vored the evolution of psychological mecha -
nisms that (1) take as input aspects of the 
individual’s own physical phenotype and 
(2) produce output functionally calibrated 
to those features. Evolved mechanisms, in 
this view, are not only attuned to recurrent 
features of the external world but can also 
be attuned to the evaluation of self. Evolved 