6 I. THEORETICAL  PERSPECTI vES AND  CONCEPT uAL uNITS
to personality psychology to help transform 
it into a more theoretically mature and ex -
planatory science.
Selection: The  “Designing” 
Evolutionary  Process
In each generation, the process of selection 
acts as a sieve (Dawkins, 1996). Variants that 
interfere with successful solutions to adap -
tive problems are filtered out. Variants that 
are tributary to the successful solution to an 
adaptive problem pass through the selective 
sieve. Iterated over thousands of genera -
tions, this filtering process tends to produce 
morphological, physiological, and psycho -
logical characteristics that interact with the 
physical, social, or internal environment in 
ways that promoted the reproduction of the 
individuals who possess the characteristics 
or the reproduction of the individual’s ge -
netic relatives (Dawkins, 1982; Hamilton, 
1964; Tooby & Cosmides, 1990a, 1990b; 
Williams, 1966). These characteristics are 
called adaptations . Adaptations that are 
psychological in nature are the subclass of 
adaptations that make up the human mind: 
evolved psychological mechanisms.
An adaptation exists in the form that it 
does because it helped to solve a specific 
problem relevant to survival or reproduc -
tion that recurred through evolutionary his -
tory. Solutions to adaptive problems can be 
direct, such as a fear of dangerous snakes 
that solves a survival problem or a desire to 
mate with fertile members of one’s species 
that helps to solve a reproductive problem. 
They can be indirect, as in a desire to as -
cend a social hierarchy, which many years 
later might give an individual better access 
to more desirable mates. Or they can be even 
more indirect, such as when a person helps a 
brother or sister, which eventually helps that 
sibling to reproduce. Our callus-  producing 
adaptations are mechanisms that evolved to 
solve the problem of potential tissue damage 
caused by repeated friction (input) by pro -
ducing additional cell layers (output) that 
protect underlying tissues in response to this 
friction. The callus example also illustrates 
the point that the adaptation is the under -
lying mechanism  responsible for producing 
calluses. Highly callused hands do not char -
acterize universal human nature; rather, the mechanisms responsible for producing cal -
luses are the adaptation shared by all typi -
cally developing members of our species. We 
elaborate on the important concept of the 
mechanism  later in this chapter.
In order for a mechanism to qualify as an 
adaptation, it must develop reliably among 
species members in all “normal” environ -
ments. Environmental events during ontog -
eny always have the potential for disrupting 
the development of a mechanism in a par -
ticular individual; thus, the genes “for” the 
mechanism do not invariantly result in its 
intact form. To qualify as an adaptation, the 
mechanism must reliably emerge in reason -
ably intact form at the appropriate time dur -
ing an organism’s life, however, and be char -
acteristic of most or all members of a species 
(with some exceptions; e.g., characteristics 
that are sex-  linked or exist in only a subset 
of members of a species due to frequent- 
dependent selection—  topics we take up later 
in this chapter).
Neither adaptations nor their products 
need be present at birth. Many develop long 
after birth. Bipedal locomotion is a reliably 
developing characteristic of humans, but 
most humans do not begin to walk until a 
year after birth. The breasts of women re -
liably develop and the testicles of males re -
liably descend, but neither process begins 
until pubertal development.
The hallmarks of adaptation are features 
that define special design —complexity, 
economy, efficiency, reliability, precision, 
and functionality (Williams, 1966). These 
qualities are conceptual criteria subject to 
empirical testing and potential falsification 
for a particular hypothesis about an adap -
tation. Because, in principle, there are an 
infinite number of alternative hypotheses to 
account for a particular constellation of find -
ings, the evaluation of a specific hypothesis 
about an adaptation is a probability state -
ment about the likelihood that the complex, 
reliable, and functional features of special 
design could not have arisen as an inciden -
tal byproduct of another characteristic or by 
chance alone (Tooby & Cosmides, 1992). As 
more and more functional features of spe -
cial design are predicted and subsequently 
documented for a hypothesized adaptation, 
each pointing to the successful solution to 
a specific adaptive problem, the alternative 