Much of the legitimacy of applying evolutionary approaches to the study of human behavior has been predicated on the existence of universal sex differences. In our species, men are on average taller (Alexander et al. 1979) and stronger than women, and die earlier and from different causes than women (Teriokhin et al. 2004). Additionally, they are generally thought to show higher variance in reproductive success than women (Barrett et al. 2002). These apparently universal sex-varying traits can be attributed to the common mammalian pattern of reproduction, in which gestation and lactation fall exclusively to women, paternity certainty is never assured, and even small amounts of paternal care are provided facultatively (Trivers 1972). As such, male fitness has, for a long time, been seen as limited by competition over access to females, and female fitness limited by access to resources that can often be acquired through males (Emlen and Oring 1977; Wrangham 1980).

Starting in the 1980s, predictions derived from parental investment theory sparked an evolutionary literature addressing human reproductive and mating strategies (reviewed in Cronk et al. 2000; Low 2000; Dunbar and Barrett 2007). Several findings emerge that suggest (or at least are interpreted as) human universals. For example, rich ethnographic and comparative studies demonstrate the prevalence of competition among men over women (Irons 1979; Betzig 1986; Chagnon 1988; Daly and Wilson 1988; Hawkes 1991), although mating competition is mediated through diverse avenues such as political office, murder, wealth accumulation, or the provision of public goods. Similar kinds of work explore how women (or their parents on their behalf) choose and compete for desirable mates (Dickemann 1979; Buss 1989; Gangestad and Simpson 2000), again through a variety of means, including cognitive preferences, dowry payments, and olfactory cues. Sex differences in mating preferences are also evident, with men tending to favor health and fecundity in their mates whereas women look for ambition and resources, as evidenced both by reported preferences (e.g., Buss 1989; Cashdan 1993) and actual behavior (e.g., Borgerhoff Mulder 1989, 1990). The exquisite sensitivity of mechanisms underlying such preferences to ecological and social circumstances (reviewed in Gangestad 2007) have helped to bring the study of human behavior into mainstream evolutionary theory, as well as to promote popular awareness of humans as yet another uniquely evolved species (e.g., Ridley 1994).

With the success of this work, there nevertheless emerged a dangerous tendency to generalize from specific observations to universal sex differences. Such generalizations are problematic for several reasons (e.g., Smith et al. 2001). First, recognizing the importance of culturally transmitted norms Boyd and Silk (2005, using Buss's 1989 data) demonstrate how cultural factors explain a great deal more of the cross-cultural variation in mate choice preferences than does gender. Second, objecting to the stereotypic portrayal of women as at the mercy of their biology and the antics of men, Hrdy (1986), Smuts (1992) and Gowaty (1997) provide cogent qualitative and quantitative support for the view that women can and do operate with agency and employ a wide array of strategies to subvert and counter the strategies of men. Third, anthropologists and others emphasize the importance of the social and ecological environment in generating variability not fixity in sex roles according to principles well established in behavioral and cultural evolutionary theory (e.g., Laland and Brown 2002).

In recent years, studies of sex differences have become more nuanced. In part, this reflects a growing awareness among psychologists and feminist scholars that gender differences have often been inflated, even derived from poor science (Shibley-Hyde 2005). Evolutionary social scientists too are using ethnographic data to emphasize the flexibility in gender roles, and the overlap in gender differences (e.g., Bliege Bird and Bird 2008). In fact, it is now indeed time to ask "are men and women really all that different?" (Borgerhoff Mulder 2004; Brown et al. 2009), and to reevaluate the extent to which sex differences in human reproductive strategies are contingent on sex differences in postzygotic investment.

In this chapter, I scrutinize the notion of universal sex differences in the reproductive strategies of men and women. My goal is cautionary. I do not argue that parental investment theory is wrong, but rather that other factors need to be taken into consideration, factors that may be of particular importance in humans. To demonstrate this point, I present empirical data on a horticultural-hunter-fisher population in Tanzania (Pimbwe) where the variation in fitness among women equals the variance in fitness of men and, quite contrary to the normative pattern, women benefit more from multiple marriages than do men. Finally, I consider how anthropologists think about the relationship between pair bonding and parental care. I finish by considering what my conclusions mean for the gap between human and nonhuman studies, the theme of this volume.

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