From an evolutionary perspective, animal mate choice depends in large part on the natural mating system of a species. The natural mating system of humans, however, seems at first glance to contain internal contradictions. On the one hand, humans show several signs of having a monogamous mating system. For example, humans are highly altricial – we have prolonged childhoods and rely heavily on extended families throughout our life spans (Alexander and Noonan, 1979). We also appear designed to form romantic pair bonds, having a dedicated neurochemistry of attachment associated with monogamy when comparisons are made across mammalian species (Fisher, 1998; Young, 2003).
On the other hand, humans seem to possess evolved design features associated with multimale/multifemale or “promiscuous” mating systems. For example, humans possess psychological and physiological adaptations to sperm competition (Baker and Bellis, 1995; Shackelford and LeBlanc, 2001), such as women's adaptive timing of extrapair copulations (Gangestad and Thornhill, 1998; Haselton and Miller, 2006), men's specialized expressions of sexual jealousy (Buss, 2000; Schützwohl, 2006), and the physical structure of the human penis serving as a semen displacement device (Gallup et al., 2003). Among men, causal sex with multiple partners is often viewed as desirable (Symons and Ellis, 1989; Oliver and Hyde, 1993), with most men agreeing to have sex with complete strangers when asked in field experiments (Clark and Hatfield, 1989). Adaptive patterns of premarital sex, extramarital sex, and mate poaching by both men and women have been documented across cultures (Broude and Greene, 1976; Schmitt et al., 2004a).