Although it has long been recognized that, around the world, adult men have larger brains on average than adult women (Pakkenberg & Voigt, 1964; Pakkenberg & Gundersen, 1997), many workers have traditionally viewed men's larger brains as simple correlates of their larger mean body masses. Other findings which suggest that the internal structure of the brains of men and women are, on average, organized differently (summarized in Kimura, 1992), and that the two sexes perform differently on certain cognitive tasks (Kimura, 1992; Falk, 1997) have traditionally been minimized with the latter being attributed largely to variations in developmental experience, as noted by Kimura (1992). Recent reports in the neurosciences, however, underscore the differences between the brains of men and women in gross volume adjusted for body size (Ankney, 1992; Falk et al., 1999), and in internal anatomy that reflects neurological wiring (Gur et al., 1999; Giedd et al., 1996b). Furthermore, convincing arguments are emerging which support the hypothesis that the neuroanatomical differences between the sexes form the substrates for their differences in average cognitive processing (Andreasen et al., 1993; Gur et al., 1999). The purpose of this chapter is to outline some of these new findings and to interpret them within an evolutionary framework.

Sex differences in brain size at equivalent body masses

Humans

Ankney (1992) plotted bivariate regression equations that relate brain weight to body height and body surface area for men and women from data provided in the literature (Ho et al., 1980a, b).