28 I. THEORETICAL  PERSPECTI vES AND  CONCEPT uAL uNITS
Over evolutionary time, those individu -
als who attended to and acted on individual 
differences in others that were adaptively 
consequential would have survived and re -
produced more successfully than those who 
were oblivious to adaptively consequential 
differences in others. Based on this analysis, 
Buss (1989b, 1996) proposed that humans 
have evolved psychological mechanisms to 
detect adaptively relevant individual differ -
ences in others.
These mechanisms would have been criti -
cal in assessing individual differences for 
the goals of mate selection, coalition forma -
tion, and dyadic alliance building. The for -
mation of these different relationships and 
the attendant adaptive problems entailed 
by them may require assessment specific -
ity; that is, different individual differences 
in the social landscape may be relevant to 
some problems and irrelevant to others. In -
dividual differences in sexual fidelity, for 
example, are more critical to assessing the 
viability of a long-term mate than a coalition 
partner (Shackelford & Buss, 1996). Despite 
some degree of domain specificity, some di -
mensions of individual differences, such as 
those captured by the five-  factor model of 
personality, may be important because they 
are relevant to a host of different adaptive 
problems, and hence may transcend the par -
ticulars of specific relationships (MacDon -
ald, 1995).
Because individual differences are so criti -
cal to solving adaptive problems, individuals 
often attempt to manipulate others’ percep -
tions and reputations of their own and com -
petitors’ standing on relevant dimensions 
of differences. In mate competition, for ex -
ample, men tend to impugn the surgency, 
agreeableness, and emotional stability of 
their rivals (Buss & Dedden, 1990). Thus, 
derogation of competitors becomes a verbal 
form of trait usage as manipulation, exploit -
ing the difference-  detecting mechanisms of 
others in the service of mate competition. 
Simultaneously, men will exaggerate their 
own positive traits in self-  presentation to a 
woman, striving to appear to fulfill char -
acteristics that she desires in a mate (Buss, 
1988b).
Whatever the origins of individual differ -
ences, whether they are or are not adaptively 
patterned, they represent important vectors in the human adaptive landscape. When the 
individual differences of others in one’s so -
cial environment are adaptively patterned, 
however, it may be especially important to 
detect and act on them, because they are 
more likely to represent coherent and hence 
predictable suites of covarying qualities 
rather than randomly varying or single- 
dimension attributes.
Ultimately, comprehensive theories of 
personality and individual differences will 
require accounts of both the adaptive and 
nonadaptive differences, as well as the 
difference-  detecting mechanisms humans 
have evolved to grapple with the varying ter -
rain of the human adaptive landscape.
Conclusions
Comprehensive theories of personality 
should aspire to include both a specifica -
tion of human nature and an account of 
the major ways in which individuals differ. 
Evolutionary psychology provides a power -
ful heuristic for the discovery of both. Since 
selection is the only known process capable, 
in principle, of producing complex organic 
mechanisms, all theories of human nature 
must at some level be anchored in the basic 
principles of evolution by selection. Theories 
of personality inconsistent with these prin -
ciples stand little or no chance of being cor -
rect (Buss, 1991).
The core of personality theory, therefore, 
must be defined by the adaptive mechanisms 
that are characteristic of most members 
of our species—  in other words, a species- 
typical human nature. The mechanisms of 
human nature cannot be fully understood 
without identifying the adaptive problems 
they were designed to solve. These adaptive 
solutions exist in the present because in the 
past they contributed, either directly or indi -
rectly, to the successful reproduction of an -
cestral humans who carried them. Modern 
humans are the end products of a long and 
unbroken chain of ancestors who succeeded 
in solving the adaptive problems necessary 
for, or that enhanced, survival and repro -
duction. As the descendants of these success -
ful ancestors, modern humans carry with 
them the psychological mechanisms that led 
to their success.