Sexual attraction based on physical appearance
Sexual attraction based on male's ability to provide resources
Imitation functions to help ensure males that they are the genetic fathers of their mate's child functions to ensure a healthy mate and one with effects of hormones (estrogen or testosterone) that indicate fertility functions to ensure females that their mates will be able to provide resources needed for the survival of their children functions to optimize the number of remaining years of fertility functions to enable children to learn culture and to profit from the experience of adults
Consider sexual attraction. The gene race is won by those who mate with fertile partners. Both sexes prefer physically attractive partners, whose good looks are signals of health. Men prefer youthful females, who have more reproductive years ahead. Many studies of attraction in a variety of societies, both in laboratory environments and by analyzing such real-world evidence as advertisements for dating introductions, confirm these predictions (Buss, 1989; Campos, Otta, & Siqueira, 2002). Details of physical attractiveness have been investigated. Bodies and faces that are more symmetrical on the left and right sides are found more attractive (by both men and women); symmetiy, because of biological mechanisms, is produced by good genetic quality (Fink & Penton-Voak, 2002; Hume & Montgomerie, 2001).
Attractive features also serve as "hormone markers," indicating higher levels of estrogen in females and testosterone in males. Research generally supports the evolutionary hypothesis that men prefer women with a low waist-to-hip ratio, an observable cue to high estrogen levels associated with greater fertility. In most studies, the ideal ratio of waist to hips is near the .7 that experts consider optimal for reproduction, but in Uganda, even larger hips attract men, probably as part of a general preference in that culture for large-bodied women (Bereczkei, 2000; Furnham, Moutafi, & Baguma, 2002). For their part, women are attracted to men whose faces show masculine bone structure features, such as a strong chin, that are produced by higher testosterone levels—especially when the women respond to researchers' questions during their most fertile phase (Fink & Penton-Voak, 2002).
Evolution selects for different traits in males and females because of different parental investment for the two sexes (Buss, 1988; Trivers, 1972). A female can produce only a limited number of children in her lifetime, because of the nine months of gestation and, in ancestral times, diminished fertility after birth when the baby is being breast-fed. Because of the greater costs in time and lost alternative opportunities with each conception, it is adaptive for the female to be particularly selective about her choice of mate. To conceive a genetically inferior child who might die or fail to cany on the genetic line would waste her reproductive opportunity, and evolution would selectively remove such unselective females from the population. The female, then, must be sure that she mates with the male who provides the highest quality genes for her investment in maternity. In addition, because of the dangers to a vulnerable child and mother, she also looks for a male who will stay to help protect them and to provide resources.
In contrast, a male's reproductive potential is much greater, since sperm cells are so much more abundant than ova and since the reduced reproductive potential after each conception is measured in hours, not years. This view probably exaggerates men's reproductive potential, relative to women (Einon, 1998), but even so, it suggests that by their nature, males are more opportunistic and females more selective in their sexual behavior. Research confirms this observation. Buss and Schmitt (1993), for example, asked undergraduates on a questionnaire how long they would wait before being willing to have sexual intercourse with a hypothetical person of the opposite sex they had just met. Females would wait for three months before becoming as willing as the man was after only one day; by three months, the man was as ready as the woman would be after two years.
The promiscuous strategy is not the only one males have, and it brings risks. His children may not survive, given the harsh environment for unprotected mothers and the danger of child-killing by competing males. Many males would leave no offspring if all adopted this promiscuous strategy. An alternative is available: to become bonded with the mother and stay to ensure that his children will survive. The increased survival of these children compensates for lost reproductive opportunities elsewhere. Some re search surveying college students finds that 99 percent of both men and women report wanting to settle down in a monogamous sexual relationship (Pedersen, Miller, Putcha-Bhagavatula, & Yang, 2002).
Since all his reproductive potential is invested in this one relationship (with perhaps a few surreptitious exceptions, if opportunity permits—to increase his reproductive potential), a new issue arises: Are the children really his, or has another man conceived them? This paternal uncertainty that the children actually are his genetic offspring has no parallel in a woman's experience (because she has carried the child during pregnancy), so sexual jealousy is greater in males. Confirming this idea, several studies find that male college students in a variety of countries report greater distress than females at the hypothetical scenario of their dating partner having sex with another man; in contrast, females were more disturbed at the prospect of their partner having an emotional relationship with another woman, theoretically because it might entice the male protector away from her (Buss, Larsen, Westen, & Semmelroth, 1992; Wiederman & Kendall, 1999). Based on reports of their actual personal experiences, though, another researcher reports no sex differences in the pain of emotional and sexual partner infidelity, either in heterosexual or homosexual respondents (C.R. Harris, 2002).
The image of the selective and committed female and the unselective, promiscuous male sexual styles is of course an oversimplification, even within evolutionary theory. Males are more willing to report infidelities on self-report measures, but actual behavioral measures show less difference between the sexes. Females, too, engage in sexual relationships outside of their primary relationship, especially when the alternative partner is superior and when they are fertile (Drigotas & Barta, 2001). Theoretical arguments explain that it sometimes is in the best reproductive interest of the female to reproduce with a more genetically fit man, even if he is not willing to stay to protect the child (Buss, 1999; Gangestad & Simpson, 1990).
The specific strategies humans use to attract a mate are less stereotyped than in lower animals, and they reflect the alternatives of seeking a long-term mate or a short-term partner (Buss & Schmitt, 1993). Based on questionnaires answered by undergraduates, Schmitt and Buss (1996) report that people use different sexual strategies to attract a partner to a short-term relationship (e.g., emphasizing sexual availability) or a long-term one (e.g., emphasizing future resource potential). Male status and dominance are more important for short-term relationships, including efforts to "poach" women who are already in a relationship with someone else; but dominance is less important for longer-term commitments, where loyalty matters (Schmitt & Buss, 2001). Among married couples, various strategies are used to retain the mate, ranging from appearance enhancement (more common in women) to threatening competitors (more common in men). Wives' youth and attractiveness and husbands' status striving and income predict mate retention (Buss & Shackelford, 1997a). Culture matters, too; when cultures have more equal status between the sexes, gender differences in sexual attraction—women's preference for high status males and men's preference for youthful females, for example— are reduced (Eagly & Wood, 1999).
Most of the data supporting male promiscuity and female commitment come from questionnaires to undergraduates, while the actual behavior of adults later shows more long-term relationships in both sexes. An alternative model suggests that both sexes are primarily drawn to long-term mating as an extension of the attachment process of earlier life. Under conditions of adversity, children are less likely to be raised in the sort of stable family that is conducive to secure attachment. This teaches them a less desirable strategy, but one that is adaptive under conditions of scarce resources, that is, to mate opportunistically, instead of counting on stable pair-bonds and high investment in par-
enting (Bereczkei, 2000). When the situation is more stable, the attachment model portrays both parents caring for children, offering an evolutionary advantage in the survival of the offspring during a long period of immaturity. Mates also provide emotional security to one another, as in the mother-child bond, and there are even parallels at the biological level: the hormone oxytocin is released both in nursing and in sexual intercourse, and it stimulates cuddling or contact comfort in both cases (Hazan & Diamond, 2000). Animal studies, using mice and voles, also point to the role of oxytocin (and vasopressin) in social attachment (LeDoux, 2002; Young, 2002). Human females, according to Shelley Taylor and colleagues (2000), respond to stress with a "friend-and-befriend" attachment and caregiving mode, mediated by oxytocin, in addition to the "fight-or-flight" mode that occurs in both sexes. This caregiving response to stress would have had evolutionary value in protecting children.
In portraying this evolutionary picture, we should not oversimplify. After all, many behaviors have no simple, obvious evolutionary advantage, and many genes combine to produce a great variety of outcomes in various individuals. Consider homosexuality, a sexual orientation influenced by a number of genes. The component genes, it has been argued, produce such traits as empathy and sensitivity, which are attractive to women and enhance fathering. When a great many of these genes are present, they produce homosexuality, which confers no reproductive advantage on the individual but which, argues E.M. Miller (2000), is consistent with evolutionary selection because of the positive contribution of the component genes on cooperation and other social traits.
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