Sexual Selection and the Evolution of Human Sex Differences

Darwin’s (1871) theory of sexual selection and the associated mechanisms of intrasexual competition (e.g., male-male competition) and intersexual choice (e.g., female choice of mates) have guided the scientific study of sex differences in hundreds of non-human species. These mechanisms and several recent advances in our understanding of the evolution and expression of sex differences in non-human species are described. The usefulness of this theory for approaching the study human sex differences is illustrated with discussion of patterns of women’s mate preferences and choices and with discussion of men’s one-on-one and coalitional competition. A comparison of these aspects of intersexual choice and intrasexual competition in humans and non-human species is provided, as is discussion of cultural variation in the expression of these behaviors. Psychologists and other social scientists have been studying human sex differences for 100 years (Acher, 1910; Woolley, 1910, 1914), and in recent decades demonstrated that many of the differences that emerge in Western cultures are found in every other culture in which they have been studied (Best & Williams, 1993; La Freniere, Strayer & Gauthier, 1984; Maccoby, 1988, 1990, 1998; Whiting & Edwards, 1988). The origin of these sex differences, however, was and remains a matter of debate. When consistent sex differences were found in the early 20 century, the proposal was their origin rested with socialization and cultural influences (e.g., Woolley, 1910). In 1914, Woolley explicitly rejected the possibility that human sex differences might be related to evolutionary and biological factors, specifically, Darwin’s (1871) sexual selection (described below). By the 1970s, there was acknowledgement that biological influences, especially sex hormones (e.g., testosterone) could influence the expression of some human sex differences, but most sex differences were still thought to be due to social and cultural influences (Maccoby & Jacklin, 1974). PSYCHOLOGICAL TOPICS 15 (2006), 2, 203-238 204 There was little, if any, consideration that human sex differences might be related to human evolutionary history until the 1980s, with the emergence of evolutionary psychology (Buss, 1989; Daly & Wilson, 1983, 1988; Symons, 1979). Even with the application of evolutionary principles in psychology and the demonstration that observed behavior and its development often result from an interaction between the genotype and experience (Caspi, McClay, Moffitt, Mill, Martin, Craig, et al., 2002), the prevailing models of the origin of human sex differences continue to emphasize socialization and cultural influences (e.g., Wood & Eagly, 2002). With this article, I will outline the logic of Darwin’s (1871) sexual selection and illustrate how these processes result in the evolution and proximate – here and now – expression of sex differences in nonhuman species. I then use these basic patterns as a means to illustrate how some human sex differences have evolved (see also Geary, 1998). Sexual Selection in Nonhuman Species Darwin and Wallace (Darwin, 1859; Darwin & Wallace, 1858) independently discovered the primary mechanisms – natural selection – that drive evolutionary change within species and result in the origin of new species. Darwin also discovered another group of mechanisms that operate within species and are the principle factors in the evolution of sex differences (Darwin, 1871). These mechanisms are called sexual selection, and involve competition with members of the same sex over mates (intrasexual competition) and discriminative choice of mating partners (intersexual choice). Although both intrasexual competition and intersexual choice can be found for both sexes, as is the case with humans, the most common mating dynamics across species involve male-male competition over access to mates and female choice of mating partners (Andersson, 1994). In the first section, I describe why this pattern is so common, and when variation from this pattern (i.e., male choice and female-female competition) is predicted to evolve. In the second and third respective sections, I provide a brief overview and a few examples of intersexual choice and intrasexual competition in nonhuman species. In the final section, I briefly discussion sex hormones as related to the evolution and proximate expression of sex differences.