The purposes of the present article are several. First,
no reasonable biologist or psychologist should disagree
with Tooby and Cosmides (1990, 1992) that humans’ psychological
mechanisms show evidence of complex design
and are largely species-specific. Nor need differential psychologists
deny the importance of the branches of psychology
that are devoted to the study of species-typical mechanisms.
However, I argue that a more up-to-date reading of
the very biology from which Tooby and Cosmides draw
their inspiration leads to a rather different view of the
extent and significance of variation. The films of functionally
significant interindividual variation need not be particularly
thin. The first purpose of this article, then, is to
review interindividual variation in nonhuman species, with
particular attention to the way that selection can allow
variation to persist even when it is relevant to fitness.