Sex and Mating

ENTERING RELATIONSHIPS AND THE TRAITS THAT ARE VALUED

PHYSIOLOGICAL MECHANISMS INVOLVED IN MATE CHOICE

BIBLIOGRAPHY

Humans in all cultures engage in various forms of mating, including marriage or committed pair-bonding as well as short-term, casual sexual relationships. Given that reproduction is at the heart of natural selection, decisions about mating are of central adaptive significance. Accordingly, mating is one of the most heavily studied areas in the biological and social sciences. Recent theoretical and empirical findings in human mating research have identified key sex differences in mate preferences, as well as neural and hormonal correlates of mating.

ENTERING RELATIONSHIPS AND THE TRAITS THAT ARE VALUED

Mate preferences may be separated into broad questions of “whether and what” (Li and Kenrick 2006). For long-term relationships, the sexes tend to be equally careful about whether to enter. When considering minimum requirements for a marriage partner, both sexes have equally high standards. However, men tend to be more eager than women to enter short-term sexual relationships, and report significantly lower standards for them, especially for one-night stands. When approached by an opposite-sex stranger who immediately makes an invitation for casual sex, 75 percent of men said yes whereas 100 percent of women said no.

To explain men’s lower short-term mating thresholds and mate preferences in general, at least two major theories have been proposed. Though often presented as competing explanations and hotly debated, sociocultural and evolutionary theories may be compatible in that the former focuses on more immediate, proximate causations, whereas the latter looks to more distal, ultimate explanations. Sociocultural theories explain sex differences by looking to social norms and the influence of larger groups. According to this view (Eagly and Wood 1999), societal norms tend to influence men to be more agentic and women to be more passive across many endeavors, including sexual behaviors. Thus, the difference in willingness to enter short-term relationships may be due to gender role differences, whereby men are socialized to be sexually autonomous and women are socialized to be sexually restrained.

According to evolutionary theorists (Buss and Schmitt 1993; Gangestad and Simpson 2000), mating psychologies may have developed in response to specific adaptive issues that long- and short-term mating pose to women and men. Men’s eagerness for sex can be traced back to differences in minimum obligatory parental investment. Whereas men are physiologically required to contribute only a few sex cells to offspring, women must provide substantial pre- and postnatal resources if offspring are to survive. Because offspring present much higher potential costs to women if they are the results of uncommitted sex, short-term relationships are reproductively less favorable for women than for men. For long-term relationships such as marriage, both sexes invest substantially in the relationship and in raising children, so both sexes may have evolved to be selective about taking on a long-term partner.

Given that a relationship will occur, an equally important consideration concerns what characteristics are valued. When considering long-term mates, men not only value physical attractiveness more than women do, but they also prioritize finding a minimum level of physical attractiveness in their partners. Women value social status more than men do, and women prioritize obtaining a minimum level of social status in long-term mates. Beyond satisfying these priorities, both sexes favor other characteristics such as kindness and intelligence, and ideally prefer a well-rounded mate. For short-term mates, both sexes tend to prioritize having a certain level of physical attractiveness before being concerned about other characteristics.

From a sociocultural perspective, women have less access to status, power, and economic resources than men do. To achieve upward mobility, women place relatively greater emphasis on status-related traits in their marriage partners. However, if the intended mating duration is short-term, then economic constraints should be less relevant and both sexes should be free to prioritize the physical attributes of their potential short-term mates, as men do for long-term mates.

Why is physical appearance prioritized over other desirable traits? Because of time constraints and variation in women’s reproductive capacity, ancestral men may have had a need to first and foremost identify long- and short-term partners who were healthy and fertile. Accordingly, men are inclined to place initial value on physical features that signal youth, sexual maturity, and fecundity. However, other characteristics are also important for long-term relationships, and once a moderate amount of physical attractiveness has been verified, more attention is paid to other desirable characteristics.

In contrast to female fertility, male fertility remains relatively constant over the life span. However, men vary in their ability to provide resources for offspring. Because ancestral men who were higher in status had better access to resources, women may have evolved to prioritize social status in long-term mates to ensure essential resources for offspring. For short-term mates, resources are less relevant, and women may value physical attractiveness in response to the adaptive issue of identifying partners with desirable heritable characteristics.

According to this “good genes” theory (Gangestad and Simpson 2000), pathogens encountered during development can lead to visible deviations from bilateral symmetry. A healthy set of genes and immune system allow a person to resist such pathogens. Because testosterone suppresses the immune system, those who simultaneously exhibit testosterone-rich features and bilateral symmetry effectively advertise having genes that are resistant to local pathogens. That is, only those with desirable good genes can afford to be loaded with potentially damaging testosterone and remain symmetrical in appearance. Indeed, male symmetry is correlated with testosterone-mediated secondary sexual characteristics such as muscularity and masculinity, and men whom women consider physically attractive exhibit more facial masculinity and bilateral symmetry. Symmetrical men are more desirable as short-term affair partners and have more sexual partners than asymmetrical men. In ancestral environments, women who mated with physically attractive men may have accrued reproductive benefits by passing on good genes to offspring.

PHYSIOLOGICAL MECHANISMS INVOLVED IN MATE CHOICE

For humans and other biparental species, trade-offs exist between parenting and mating, and individual hormonal profiles likely mediate both male and female reproductive strategies along these lines (Clutton-Brock 1991). In order for conception to occur during a woman’s menstrual cycle, a set of hormonal conditions must be present. First, a mid-cycle rise in luteinizing hormone must occur, signaling impending egg maturation. Second, the ovarian hormones estradiol and progesterone must be at optimal levels, with estradiol rising prior to ovulation and progesterone hitting peak levels after ovulation has occurred. Natural selection may have shaped women’s mating psychologies to parallel such changes in physiology.

For females, it would be reproductively ideal to find mates who can provide both material and genetic benefits to offspring; however, obtaining both sets of features in one male is difficult. Therefore, most women may need to make strategic trade-offs by selecting long-term partners who are higher in investment potential than sexual attractiveness. Moreover, these trade-offs may have selected for a dual-mating psychology in which women seek primary partners who provide investment while obtaining better genes through extra-pair mating (i.e., mating with individuals other than one’s primary partner).

Indeed, around ovulation, female sexual desire becomes stronger and the number of sexual fantasies increases. However, these are directed not toward primary partners, but toward potential affair partners. This is particularly true if the primary partner is less physically attractive and lacks indicators of genetic fitness, including strength, social dominance, and symmetry. When near ovulation, women prefer masculine and symmetrical faces in men, and the scent of symmetrical men. For women who are in a steady relationship, in-pair sex tends to occur consistently across the cycle, whereas extra-pair sex occurs significantly more often on high-fertility days.

Males also face strategic trade-offs involving differential allocation of effort to mating or parenting. The resolution of these trade-offs depends on cues from the environment. Men tend to allocate more effort to mating to the extent to which they possess indicators of good genotypic quality. When men do not have the attributes that make them attractive to females, or otherwise face limited sexual opportunities, they tend to invest more heavily in a single mate’s offspring. For example, African tribal evidence shows that men of high status have more wives and spend less time on parenting than men of low status.

Testosterone is a steroid hormone that plays a key role in mediating trade-offs in male mating strategies. Higher testosterone levels are associated with greater promiscuity and less parenting. Men with higher testosterone are less likely to have ever married and, if they do marry, are more likely to engage in extramarital sex. The testosterone of men who marry falls as they transition from bachelor to husband, and testosterone remains low among stably married men.

At the neural level, the hormones oxytocin and vasopressin appear to be involved in the formation of pair bonds between males and females. Recent insight into the neuroendocrine basis of monogamous pair bond formation comes from the comparative study of two rodent species. The prairie vole and the meadow vole are two very closely related species that differ markedly in mating strategies. Male and female prairie voles form lifetime pair-bonds, whereas meadow voles are highly promiscuous. During copulation, oxytocin and vasopressin are released in the brain. Oxytocin facilitates affiliation and partner preference behavior in female prairie voles, and vasopressin facilitates affiliative and parenting behavior in male prairie voles. Exogenous injections of vasopressin to the male prairie vole and oxytocin to the female facilitate mate preference behavior without the occurrence of mating. Likewise, blocking the receptors for these hormones will prevent pair-bonding from occurring after copulation. In contrast, similar injections do not influence social behavior in the meadow vole.

Though human brains release oxytocin and vasopressin during sex, relatively little is known about their precise functions in humans. Nevertheless, findings from other species provide a starting point, and ongoing efforts to understand the changes in gene expression for oxytocin and vasopressin receptor density may help to clarify individual differences in human mating strategies.

SEE ALSO Evolutionary Psychology; Romance

BIBLIOGRAPHY

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Thornhill, Randy, and Steven W. Gangestad. 1994. Fluctuating Asymmetry and Human Sexual Behavior. Psychological Science 5: 297-302.

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Trivers, Robert L. 1972. Parental Investment and Sexual Selection. In Sexual Selection and the Descent of Man, 1871-1971, ed. Bernard Campbell, 136-179. Chicago: Aldine.

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Norman P. Li

Kristina M. Durante