Evolutionary Psychology ch 6_1-6.pdf

ChaPter 6 Short-Term Sexual Strategies Learning Objectives Afer studying this chapter, the reader will be able to: ■ list the adaptive benefts to men of short-term mating from an evolutionary perspective. ■ list the costs of short-term mating for men. ■ identify the adaptive problems men must...

ChaPter 6 Short-Term Sexual Strategies Learning Objectives Afer studying this chapter, the reader will be able to: ■ list the adaptive benefts to men of short-term mating from an evolutionary perspective. ■ list the costs of short-term mating for men. ■ identify the adaptive problems men must solve when pursuing short-term mating. ■ name three empirical fndings that support the hypothesis that men have an evolved short-term mating strategy. ■ specify the fve major hypotheses about the adaptive benefts to women of short-term mating. ■ analyze the evidence for contexts that infuence whether women pursue short-term mating. [Women] not rarely run away with a favoured lover. . . . We thus see that . . . the women are not in quite so abject a state in relation to marriage as has often been supposed. They can tempt the men they prefer, and sometimes can reject those whom they dislike, either before or after marriage —Charles Darwin (1871) Imagine an attractive person of the opposite sex walking up to you on a college campus and saying, “Hi, I’ve been noticing you around town lately, and I fnd you very attractive. Would you have sex with me?” How would you respond? If you are like 100 percent of the women in one study, you would give an emphatic no. You might be ofended, insulted, or just plain puzzled by the request. But if you are like the men in that study, the odds are good that you would say yes— as did 75 percent of those men (Clarke & Hatfeld, 1989). As a man, you would most likely be fattered by the request. Subsequent research found that men report more willingness to accept sexual ofers from attractive than unattractive women; women are willing to accept sexual ofers from men high in socioeconomic status and high in attractiveness if the context involves some level of emotional intimacy rather than just pure sex (Greitemeyer, 2005). A study of German, Italian, and U.S. participants also found that attractiveness mattered for both sexes (Schützwohl, Fuchs, McKibben, & Shackelford, 2009). For men, 65 percent indicated some level of likelihood of having sex if the woman was slightly unattractive; 79 percent if she was moderately attractive; and 82 percent if she was extremely attractive. For women, 5 percent indicated some level of likelihood of the man was slightly unattractive; 13 percent if he was moderately attractive; and 24 percent if he was extremely attractive. A study in Denmark revealed that relationship status matters (Hald & Høgh-Olesen, 2010). For Danish men in a relationship, only 18 percent agreed to go to bed with the female confederate, whereas 59 percent not in a relationship agreed to go to bed with her (the comparable fgures for women were 4 and 0 percent, respectively). The idea that men and women react diferently when it comes to casual sex may not be surprising. That this gender diference is both extremely large and universal across cultures may be more surprising. Theories in evolutionary psychology provide a principled basis for predicting this diference. ChALLENGES OF SEx AND MATiNG 160 Theories of Men’s Short-Term Mating We begin by considering theories of short-term mating. First, we will look at the adaptive logic of men’s short-term mating and why it would loom larger in men’s than in women’s psychological repertoires. Second, we examine the potential costs that men might incur from short-term mating. Third, we explore the specifc adaptive problems that men must solve if they are to successfully pursue short-term mating. Adaptive Benefts for Men of Short-Term Mating Trivers’s (1972) theory of parental investment and sexual selection, described in Chapter 4, provides a powerful basis for expecting sex diferences in the pursuit of short-term mating: Men, more than women, are predicted to have evolved a greater desire for casual sex. The same act of sex that causes a woman to invest 9 months of internal gestation obligates the man to practically no investment. Over a 1-year period, an ancestral man who managed to have short-term sexual encounters with dozens of fertile women would have caused many pregnancies. An ancestral woman who had sex with dozens of men in the course of the same year could produce only a single child (unless she bore twins or triplets). See Box 6.1 for a discussion of function and benefcial efects of short-term mating. The reproductive benefts for men who successfully implemented a short-term mating strategy would have been direct: an increase in the number of ofspring produced. A married man with two children, for example, could increase his reproductive success by a full 50 percent by one short-term copulation that resulted in conception and birth. This beneft assumes, of course, that the child produced by such a brief union would have survived, which would have depended in ancestral times on a woman’s ability to secure resources through the father or through other means, such as by herself, through kin, or through other men. Historically, men appear to have achieved increases in reproductive success mainly through increases in the number of sexual partners, not through increases in the number of children per partner (Betzig, 1986; Dawkins, 1986). Potential Costs of Short-Term Mating for Men Short-term sexual strategies, however, carry potential costs for men. Over evolutionary time, men risked (1) contracting sexually transmitted diseases, a risk that increases with the number of sex partners; (2) acquiring a social reputation as a “womanizer,” which could impair their chances of fnding a desirable long-term mate; (3) lowering the chances that their children would survive due to lack of paternal investment and protection; (4) sufering violence at the hands of jealous husbands or boyfriends if the women were married or mated; (5) sufering violence at the hands of the father or brothers of the women; and (6) risking retaliatory afairs by their wives and the potential for a costly divorce (Buss & Schmitt, 1993; Daly & Wilson, 1988; Freeman, 1983). Given the large potential adaptive advantages of short-term mating for men in the currency of increased ofspring production, selection might have favored a short-term mating strategy despite these costs. More precisely, selection would favor adaptations in men to pursue short-term mating in circumstances in which the costs and risks could be avoided or minimized. 6 ShORT-TERM SExuAL STRATEGiES Adaptive Problems Men Must Solve When Pursuing Short-Term Mating Ancestral men who pursued a short-term sexual strategy confronted a number of specifc adaptive problems—partner number or variety, sexual accessibility, identifying which women were fertile, and avoiding commitment. 6.1 Functions Versus Benefcial Efects of Short-Term Mating Short-term mating may have benefcial efects that are diferent from the original function. For example, “gaining backstage access to famous musicians at a rock concert by having sex with the road manager” may be a benefcial efect of short-term mating but could not have been an original function of such mating. Rock concerts are modern inventions and are not part of the selective environment in which humans evolved. Of course, this does not preclude “exchange of sex for social access, benefcial position, or privilege” as a more abstract function of short-term mating (Meston & Buss, 2009). For a beneft to qualify as a proper evolved function of short-term mating means (1) that there was recurrent selection pressure over human evolutionary history such that the beneft was predictably reaped by those who engaged in short-term mating in specifc circumstances; (2) that the costs in ftness currencies of pursuing short-term mating were outweighed by the benefts in the contexts in which they were pursued; and (3) that selection favored the evolution of at least one psychological mechanism specifcally designed to promote short-term mating in specifc circumstances. Because we cannot go back in time, we must use various standards of evidence for inferring the evolution of psychological mechanisms specifcally designed to promote short-term mating. Among the criteria we can adopt are: (1) Do people in most or all cultures engage in short-term mating under particular conditions when not physically or socially constrained from doing so? (2) Are there specifc contexts that predispose men and women to engage in shortterm mating that would imply the existence of psychological mechanisms sensitive to those contexts? (3) On the basis of our knowledge of ancestral environments, is it reasonable to infer that those specifc contexts would have provided recurrent opportunities to engage in short-term mating? (4) Was a potential beneft likely to be received by a woman or a man engaging in short-term mating in those contexts? Given the prevalence of short-term mating across all known cultures, the prevalence of infdelity in plays and novels dating back centuries, the evidence for human sperm competition (Baker & Bellis, 1995), and the expressed desire for sexual variety by both genders, it is reasonable to infer that ancestral conditions would have permitted recurrent opportunities for women and men to beneft from short-term mating in some circumstances. The Problem of Partner Number or Variety Successful pursuit of short-term mating requires an adaptation that is motivational, something that would impel men toward a variety of sex partners. One solution is the desire for sex with a variety of diferent women (Symons, 1979). A second specialized adaptation is a relaxation of standards for an acceptable short-term partner. A third predicted adaptation is to impose minimum time constraints—that is, to let little time elapse before seeking sexual intercourse. The Problem of Sexual Accessibility Advantages would accrue to men who focused their mating eforts toward women who were sexually accessible. Time, energy, and courtship resources devoted to women who are unlikely to consent to sex would interfere with the successful pursuit of short-term mating. Specialized adaptations for solving the problem of sexual accessibility might occur in the form of men’s short-term mate preferences. Men might disfavor women who show signs of being prudish, conservative, or low in sex drive, since these qualities might suggest lower odds of successful short-term mating. Clothing and behavior that signal potential sexual availability might be desired by men in short-term mates. The Problem of Identifying Which Women Are Fertile A clear evolutionary prediction is that men seeking short-term mates would prefer women who displayed cues correlated with fertility. A maximally fertile woman would have the highest 161 ChALLENGES OF SEx AND MATiNG 162 probability of getting pregnant from a single act of sex. In contrast, men seeking long-term mates might be predicted to prefer younger women of higher reproductive value, because such women will be more likely to reproduce in the future (see Chapter 5 for a discussion of the distinction between fertility and reproductive value). This distinction—fertility versus reproductive value— does not guarantee that selection will have fashioned two diferent standards of attraction in men, one for casual sex and another for a marriage partner. The key point is that this distinction can be used to generate a hypothesis about shifts in age preferences, which we can then test. The Problem of Avoiding Commitment Men seeking short-term mates are predicted to avoid women who might demand serious commitments or investments before consenting to sex. The larger the investment in a particular woman, the fewer the number of sex partners a man can succeed in attracting. Women who require heavy investment efectively force men into a long-term mating strategy. Men seeking short-term mates, therefore, are predicted to shun women who demand commitments or heavy investments before agreeing to sex (Jonason & Buss, 2012). Evidence for an Evolved Short-Term Mating Psychology in Males Casual sex typically requires the consent of both sexes. At least some ancestral women must have practiced the behavior some of the time, because if all women historically had mated monogamously for life with a single man and had no premarital or extramarital sex, opportunities for casual sex with consenting women would have vanished (Smith, 1984). The exception, of course, would occur in the context of coerced sex—a phenomenon that is extremely serious, illegal in most countries, and something that evolutionary psychology can help to understand and potentially prevent—issues we will explore in depth in Chapter 11. Physiological Evidence for Short-Term Mating Existing adaptations in our psychology, anatomy, physiology, and behavior refect the scoring of prior selection pressures. Just as the modern fear of snakes reveals an ancestral hazard, so our sexual anatomy and physiology reveal ancient short-term sexual strategies. Testicle Size One clue comes from the size of men’s testicles. Large testes typically evolve as a consequence of intense sperm competition—when the sperm from two or more males occupy the reproductive tract of one female at the same time because she has copulated with two or more males (Baker & Shackelford, 2018; Short, 1979; Smith, 1984). Sperm competition exerts a selection pressure on males to produce large ejaculates containing numerous sperm. In the race to the valuable egg, the larger, sperm-laden ejaculate has an advantage in displacing the ejaculate of other men inside the woman’s reproductive tract. Men’s testes size, relative to their body weight, is far greater than that of gorillas and orangutans. Male testes account for .018 percent of body weight in gorillas and .048 percent in orangutans (Short, 1979; Smith, 1984). In contrast, human male testes account for .079 percent of men’s body weight, or 60 percent more than that of orangutans and more than four times that of gorillas, corrected for body size. Men’s relatively large testes provide one piece of evidence that women in human evolutionary history sometimes had sex with more than one man within a time span of a few days. This size of testes would have been unlikely to evolve unless there was sperm competition. And it suggests that both sexes pursued short-term mating some of the time. But humans do not possess 6 ShORT-TERM SExuAL STRATEGiES the largest testes of all the primates. Human testicular volume is substantially smaller than that of the highly promiscuous chimpanzee, whose testes account for .269 percent of its body weight, more than three times the percentage for men. Our human ancestors, according to this evidence, rarely reached the chimpanzee’s extreme of relatively indiscriminate sex. To get a concrete feel for the diferences in sexuality between chimps and humans, Wrangham (1993) summarized data from a variety of studies on the estimated number of male copulation partners that females from a variety of primate species experienced per birth. The highly monogamous gorilla females averaged only one male sex partner per birth. Human females were estimated to have 1.1 male sex partners per birth, or nearly 10 percent more sex partners than gorillas. In contrast, baboon females had eight male sex partners per birth; bonobo chimp females had nine male sex partners per birth; and common chimpanzee females (Pan troglodytes) had 13 male sex partners per birth. Thus, the behavior that leads to sperm competition—females having sex with a variety of males—appears to accord well with the evidence on sperm volume. Humans show higher levels of sperm competition than the monogamous gorillas but far lower levels of sperm competition than the more promiscuous chimps and bonobos. Variations in Sperm Insemination Another clue to the evolutionary existence of casual mating comes from variations in sperm production and insemination (Baker & Bellis, 1995). In a study to determine the efect on sperm production of separating mates from each other, 35 couples agreed to provide ejaculates resulting from sexual intercourse from condoms. The partners in each couple had been separated for varying intervals of time. Men’s sperm count went up dramatically with the increasing amount of time the couple had been apart since their last sexual encounter. The more time spent apart, the more sperm the husbands inseminated in their wives when they fnally did have sex. When the couples spent 100 percent of their time together, men inseminated 389 million sperm per ejaculate, on average. But when the couples spent only 5 percent of their time together, men inseminated 712 million sperm per ejaculate, almost double the amount. The number of sperm inseminated, according to the authors of the study, increases when other men’s sperm might be inside the wife’s reproductive tract at the same time due to the opportunity provided for extramarital sex. The increase in sperm insemination upon being reunited did not depend on the time since the man’s last ejaculation. Even when the man had masturbated to orgasm while away from his wife, he still inseminated more sperm on being reunited if he had been away from her a long time. The increase in sperm inseminated by the husband after prolonged separation ensures that his sperm will stand a greater chance in the race to the egg by crowding out or displacing a possible interloper’s sperm. Psychological Evidence for Short-Term Mating In this section, we consider the psychological evidence for short-term mating—the desire for sexual variety, the amount of time that elapses before a person seeks sexual intercourse, the lowering of standards in short-term mating, the nature and frequency of sexual fantasies, and the “closing time phenomenon.” Desire for a Variety of Sex Partners One psychological solution to the problem of securing sexual access to a variety of partners is lust: Men have evolved a powerful desire for sex. Men do not always act on this desire, but it is a motivating force: “Even if only one impulse in a thousand is consummated, the function of lust nonetheless is to motivate sexual intercourse” (Symons, 1979, p. 207). 163 ChALLENGES OF SEx AND MATiNG 164 Figure 6.1 “Ideally, How Many Diferent Sexual Partners Would You Like to Have in the Next Month?” Total sample size: 16,288. Source: Data from International Sexuality Description Project, courtesy of David P. Schmitt. To fnd out how many sexual partners people desire, researchers asked unmarried U.S. college students to identify how many sex partners they would ideally like to have within various time periods, ranging from the next month to their entire lives (Buss & Schmitt, 1993; Kennair et al., 2009; Schmitt et al., 2003). The results from a large-scale cross-cultural study are shown in Figure 6.1 (Schmitt et al., 2003). In every culture in every region of the world, a substantially larger percentage of men than women desire more than one sex partner over the next month. Norwegian culture provides an especially interesting test case for these sex diferences, since it is a culture with a high degree of gender equality (Kennair et al., 2009). Norwegian women desire roughly 2 sex partners over the next year; Norwegian men desire seven. Over the next 30 years, Norwegian women desire roughly 5 sex partners; men desire nearly 25. Some psychologists argue that increased gender equality should result in a reduction or elimination of sex diferences (Eagly & Wood, 1999). This clearly has not happened in Norway or in any other culture examined so far. Another study analyzed 48 “private wishes” ranging from “to be with God when I die” to “to make a lasting contribution through creative work” (Ehrlichman & Eichenstein, 1992). The largest sex diference by far was found for one wish: “to have sex with anyone I choose.” In another study that asked 676 individuals to estimate their frequency of experiencing sexual desire, the average man estimated 37 times per week, whereas the average woman estimated 9 times per week (Regan & Atkins, 2006). And in a cross-cultural study of 16,288 people from 10 major world regions, including six continents, 13 islands, 27 languages, and 52 nations, men expressed a desire for a larger number of sex partners than women did in all countries (Schmitt et al., 2003). From the small island of Fiji to the large island of Taiwan, from the north of Scandinavia to the south of Africa, in every island, continent, and culture, men expressed a substantially greater desire than did women for a variety of diferent sex partners. Time Elapsed Before Seeking Intercourse Another psychological solution to the problem of gaining sexual access to a variety of partners is to let little time elapse between meeting a desired potential partner