Mate Selection

Taking a still broader perspective, we can ask, "How does mate selection in humans compare with mate selection in other animals?" Looking across many animal species, evolutionary biologists have uncovered general principles that may help clarify some of the particulars of human mate selection.

At the broadest level, the theory of inclusive fitness suggests all animals are selected to behave in ways that, on average, benefit others sharing their genes (siblings and cousins as well as their own offspring). Sexual selection refers to a form of natural selection favoring characteristics that assist in attracting mates (e.g., peacock's feathers) or in competing with the same sex (e.g., rams' horns). Across species, females are more likely to be the selectors, and males are more likely to be found banging their heads against one another to win females' attention. According to differential parental investment theory, the sex with the initially higher investment in the offspring—generally the female—has more to lose from a poor mating choice and therefore demands more before agreeing to mate (Trivers 1972). In species in which males make the larger investment (e.g., by caring for the eggs and young, as in seahorses), males tend to be more selective about their mates (Daly and Wilson 1983). In mammals, the normal discrepancy between males and females is especially pronounced, because females carry the young inside their bodies and nurse them after birth. Male mammals can reproduce with little cost, and, frequently, the male's direct input does not go beyond the simple act of copulation. In such species, males tend to be nonselective about their mates, whereas females demand evidence of superior genetic potential before mating and will often mate only with males who have demonstrated superior capabilities. Humans also sometimes have sexual relations within less committed relationships, in the typical mammalian mode. Under those circumstances, males are less selective (Kenrick et al. 1990). Unlike most mammals, however, humans tend to form long-term pair-bonds, in which males invest many resources in the offspring. Under those circumstances, men's selectivity about mates approaches that of women (Kenrick et al. 1990).

Men and women make different contributions to the offspring. Women contribute their bodies, through internal gestation and nursing, and men consequently value indications of fertility including healthy appearance and a waist-hip ratio characteristic of youthful sexual maturity (Cunningham, Druen, and Barbee 1997). On the other hand, men primarily contribute their genes and indirect resources such as money and shelter. Women could appraise a man's genetic potential from physical attractiveness and position in a dominance hierarchy (Gangestad, Thornhill, and Yeo 1994). His ability to provide resources could be gauged indirectly by his ambition and directly by his social status and acquired wealth (Buss and Barnes 1986; Daly and Wilson 1983). Even with these differential tendencies, humans often cooperate in raising their offspring. Hence a number of characteristics should be (and are) desired by both sexes, such as agreeableness, kindness, and faithfulness (Buss 1989; Kenrick et al. 1990). People are not presumed to consciously calculate their genetic self-interest, but like all animals, to have inherited certain preferences that helped their ancestors reproduce successfully.