Because age and sex constitute the only distinct anatomical and physiological morphs (types) of the human species, universal sex differences ought to be expected.
According to Darwinian theory, the most numerous sex differences are likely to be found in the domains of sexuality and reproduction.
We first briefly review the basic model of the adaptationist program of modern Darwinian psychology.
We then present evidence suggesting substantial sex differences in the following domains of sexual behavior:
Mate preferences, interest in casual sex, interest in partner variety, jealousy, fantasy, sexual "plasticity" and magnitude of intrinsic sexual motivation.
We then propose a program for research and explanation of sex differences that invokes both proximate and ultimate variables where appropriate.
This program is based in modern Darwinian theory, neuroendocrinology, human genetics, and social and behavioral sciences.
We conclude by considering sociopolitical implications of research on sex differences.