The unusual women of Mpimbwe: Why sex differences in humans are not universal

Abstract

Parental investment theory provides a strong basis for generalizations about how male and female mating strategies might vary, and has generated a large number of successful predictions regarding gender differences in human reproductive strategies. There are, however, many situations in which traditional sex roles are not observed, and behavioral ecologists are beginning to determine how and why this might be. In this chapter, I explore the implications of generalizations about universal sex differences for our understanding of gender differences in sexual and reproductive strategies of humans. First, I examine recent work within behavioral ecology on the status of parental investment as a determinant of sex differences in reproductive strategies. Second, I summarize analyses of reproductive strategies in a rural forager-horticultural population in western Tanzania where variance in women's reproductive success is not significantly different from that of men and where women use serial matings rather more effectively than do men to outcompete their competitors, to show that key sex differences predicated on the mammalian pattern of parental investment are not necessarily observed. Third, I broaden this discussion of an obvious ethnographic exception to examine the relationship between human pair bonds and parental investment, to show again that sex differences in parental investment provide only a partial story. The implications of these observations for claims of universal sex differences and the gap between studies of human and nonhuman reproductive strategies are discussed in the conclusion.