The biological irony of the double standard is that males could not have been selected for promiscuity if historically females had always denied them opportunity for expression of the trait.
—ROBERT SMITH, Sperm Competition and the Evolution of Mating Systems
IMAGINE THAT AN attractive person of the opposite sex walks up to you on a college campus and says: “Hi, I’ve been noticing you around town lately, and I find you very attractive. Would you go to bed with me?” How would you respond? If you are like 100 percent of the women in one study in Florida, you would give an emphatic no. You might be offended, insulted, or plain puzzled by the request out of the blue. But if you are a man, the odds are 75 percent that you would say yes.1 You would most likely feel flattered by the request.
Perhaps there is something odd about the culture in Florida, so other researchers sought to replicate these findings. Particularly informative are studies based in sexually liberal cultures such as France and Denmark. In France, for example, women are increasingly consuming pornography, having sex with men they meet on the Internet, and having a larger number of casual sex partners. When French women were approached with a request for sex, however, only 3 percent agreed to have sex with the approaching stranger if he was highly attractive, and 0 percent if he was only average.2 In contrast, a full 83 percent of French men consented to sex with an attractive woman, and 60 percent agreed if she was average in attractiveness. Studies carried out in Denmark produced similar results, although men in committed romantic relationships were considerably less likely to consent to sex than were single men.3 Men and women react differently when it comes to casual sex, and willingness to consent to sex with strangers differs strongly by gender across cultures.
Casual sex typically requires mutual consent, however. Ancestral men could not have had temporary hookups unaided. At least some ancestral women must have been sexually willing or eager some of the time, barring coerced sex (a topic explored in depth in Chapter 7). If all women had mated for life with a single man—and in most traditional cultures most postpubertal females are married—opportunities for casual sex would seem to have vanished.4 So short-term sex would have meant that women were engaging in extramarital affairs. One of the keys to ancestral women’s extramarital sexual opportunities was a lapse in mate guarding. Hunting opened windows of opportunity, because men went off for hours, days, or weeks to procure meat.
In spite of the prevalence and evolutionary significance of casual sex, until recently most scientific research on human mating has centered on long-term mating. The typically transient and secretive nature of casual sex makes it difficult to study. In Alfred Kinsey’s classic research on sexual behavior, for example, questions about extramarital sex prompted many people to refuse to be interviewed altogether. Among those who did consent to an interview, many declined to answer questions about extramarital sex.
Our relative ignorance about casual sexual encounters also reflects deeply held values. Many of us shun the promiscuous and scorn the unfaithful because such people often interfere with our own sexual strategies. From the perspective of a married woman or man, for example, the presence of promiscuous people endangers marital fidelity. From the perspective of a single woman or man seeking marriage, the presence of promiscuous people lowers the likelihood of finding someone willing to commit. We derogate male short-term strategists as cads, womanizers, or “man whores” because we want to discourage casual sex, at least among some people. People also derogate women by “slut-shaming.” Casual sex is in many ways still a taboo topic. But it fascinates us. We must look closer and ask why it looms so large in our mating repertoire.
Although women and men alike have the whole repertoire of mating strategies—long-term mating, short-term mating, mate poaching, infidelity, and so on—there do exist somewhat stable individual differences, sometimes called sociosexual orientation. Some people are strongly inclined to long-term, high-investment mating. They want sex in the context of a loving committed relationship. Others are more inclined to short-term mating; casual sex without love or encumbering commitment feels fine to them. Whereas long-term maters search for “the one and only,” short-term maters thrive on sexual variety and tend to experience a larger number of sex partners. So we turn now to the clues present in a deep-time evolutionary history of short-term mating.
Physiological Clues to Sexual Strategies and the Mystery of Female Orgasm
Existing adaptations in our psychology, anatomy, physiology, and behavior reflect prior evolutionary selection pressures. Just as our current fear of snakes betrays an ancestral hazard, so our sexual anatomy and physiology reveal an ancient story of short-term sexual strategies. Important clues to that story have come to light through careful studies of men’s testes size, ejaculate volume, variations in sperm production, and a possible function of female orgasm.
Large testes typically evolve as a consequence of intense sperm competition when the sperm from two or more males occupy the reproductive tract of the female at the same time.5 Sperm competition exerts a selection pressure on males to produce large ejaculates containing numerous sperm. In the race to the valuable egg, the more voluminous sperm-laden ejaculate has an advantage in displacing the ejaculate of other men inside the woman’s body.
The testes size of men, relative to their body weight, is far larger than that of gorillas and orangutans. Male testes account for 0.018 percent of body weight in gorillas and 0.048 percent in orangutans. In contrast, men’s testes account for 0.079 percent of body weight—60 percent more than that of orangutans and more than four times that of gorillas. Men’s relatively large testes provide one solid 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. The comment made in many cultures that a man has “big balls” may be a metaphorical expression with a literal referent. But humans do not possess the largest testes of all the primates. Human testicular volume is substantially smaller than that of the highly promiscuous chimpanzee, whose testes account for 0.269 percent of body weight, which is more than three times the percentage of men. These findings suggest that our human ancestors rarely reached the chimpanzee’s extreme of promiscuity.6
Another clue to the evolutionary existence of casual mating comes from variations in sperm production and insemination.7 In a study to determine the effect of separating mates from each other on sperm production, thirty-five couples agreed to provide ejaculates resulting from sexual intercourse, either from condoms or from the flowback (the gelatinous mass of seminal fluid spontaneously ejected by a woman at various points after intercourse).8 All the couples had been separated from each other for varying intervals of time.
Men’s sperm count increased dramatically with the increasing amount of time the couple had been apart. The more time spent apart, the more sperm the husbands inseminated in their wives when they finally had sex. When the couples spent 100 percent of their time together, men inseminated only 389 million sperm per ejaculate. But when the couples spent only 5 percent of their time together, men inseminated 712 million sperm per ejaculate, or almost double the amount. Sperm insemination increases when other men’s sperm might be inside the wife’s reproductive tract at the same time, as a consequence of the opportunity provided for extramarital sex by the couple’s separation. This increase in sperm is precisely what would be expected if humans had an ancestral history of some casual sex and marital infidelity.
The increase in sperm insemination by the husband upon prolonged separation gives his sperm a greater chance in the race to the egg by crowding out or displacing any interlopers’ sperm. A man appears to inseminate just enough sperm to replace the sperm that have died inside the woman since his last sexual episode with her, thereby “topping off” his mate to a particular level to keep the population of his sperm inside her relatively constant. Men appear to carry a physiological mechanism that elevates sperm count when their wives may have had opportunities to be unfaithful.
The physiology of women’s orgasm provides another clue to an evolutionary history of short-term mating. Once it was thought that a woman’s orgasm functions to make her sleepy and keep her reclined, thereby decreasing the likelihood that sperm will flow out and increasing the likelihood of conceiving. But if the function of orgasm were to keep the woman reclined so as to delay flowback, then more sperm would be retained when flowback is delayed. That does not happen. Rather, there is no link between the timing of the flowback and the number of sperm retained.9
Women on average eject roughly 35 percent of the sperm within thirty minutes of the time of insemination. If the woman has an orgasm, however, she retains 70 percent of the sperm and ejects only 30 percent. Lack of an orgasm leads to the ejection of more sperm. This evidence is consistent with the theory that women’s orgasm functions to suck up the sperm from the vagina into the cervical canal and uterus, increasing the probability of conception.
A key to the mystery of female orgasm appears to be the link between female orgasm and sperm retention, combined with a hidden side of female sexuality—women’s sexual infidelities. Kinsey found that women were almost twice as likely to achieve orgasms with their affair partners as with their husbands. A British study found that women have more frequent high-sperm-retention orgasms—those that occur within two minutes after the male orgasm—with their affair partners than with their husbands. The clincher, however, may be the timing of lunchtime romance at the No-Tell Motel: women who have affairs appear to time their sexual liaisons to coincide with the most fertile phase of their cycle—prior to or at ovulation. Indeed, the rate of sexual intercourse with an affair partner during peak fertility is three times as high as the rate during the low-fertile post-ovulatory phase.10
Female orgasm may function as a selection device to choose which man will end up fertilizing her eggs, a man who is not necessarily her husband. Women are more orgasmic with regular mates who have good genetic quality, as indexed by anatomical measures of symmetry and judgments of physical attractiveness. But if they are having affairs, women preferentially choose affair partners of high genetic quality and then experience more frequent sexual orgasms in the context of their liaisons. For women having affairs, orgasm may facilitate a mating strategy of getting the best of both worlds—investment from one man who provides parenting and resources for her children and good genes from another man who provides little investment but who increases the genetic quality of her children.
Although my reading of scientific evidence leads me to conclude that the female orgasm shows at least some design hallmarks of adaptation, skeptics of this conclusion—advocates of the hypothesis that female orgasm is a non-adaptive by-product analogous to male nipples—still have plenty of ammunition to work with. No current adaptationist hypothesis, for example, can explain why there appear to be huge individual differences and cultural differences in the occurrence of female orgasm. Further-more, it is entirely possible that the female orgasm is neither 100 percent adaptation nor 100 percent by-product, but rather contains certain features that are by-products combined with some adaptive modification.
This may not be good news for the husbands of women seeking sex outside marriage, but it suggests that women have evolved strategies that function for their own reproductive benefit in the context of extramarital affairs, perhaps by securing good genes from one man and investment from their regular mate.
In addition to this anatomical and physiological evidence, psychological and behavioral evidence also suggest a long evolutionary history of casual, short-term mating.
Lust
Psychological adaptations point to a human evolutionary past of casual sex. Because the adaptive benefits of temporary liaisons differ for each gender, however, evolution has forged different psychological mechanisms for men and women. For ancestral men, the primary benefit of casual sex was a direct increase in the number of offspring. Men consequently faced a key adaptive problem—how to gain sexual access to a variety of women.
One psychological solution to the problem of securing sexual access to a variety of partners is old-fashioned lust. Men have evolved a powerful desire for sexual access to a variety of women. When President Jimmy Carter told a reporter that he “had lust in his heart,” he expressed honestly a universal male desire for sexual variety. 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.”11
To find how many sexual partners people desire, the study of short-term and long-term mating asked unmarried 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 lifetime.12 Men desired more sex partners than women at each of the different time intervals. Within the next year, for example, men stated on average that ideally they would like to have more than six sex partners, whereas women said that they would like to have only one. Within the next three years, men desired ten sex partners, whereas women wanted only two. In their lifetime, men on average wanted to have eighteen sex partners and women only four or five.
David Schmitt’s massive study of fifty-two different cultures located on six continents and thirteen islands uncovered the same pattern.13 Norwegian culture provides an especially interesting test case for these sex differences, since it is a culture with a high degree of gender equality.14 Over the next thirty years, Norwegian women desired roughly five sex partners; Norwegian men desired roughly twenty-five. Some psychologists argue that increased gender equality will reduce or eliminate these and other sex differences.15 This clearly has not happened in Norway or in any other culture studied so far. Men’s inclination to count their “conquests” and to “put notches on their belt,” long incorrectly attributed in Western culture to male immaturity or masculine insecurity, in fact reflects an adaptation to motivate brief sexual encounters.
Studies of sex drive reveal similar gender differences. The most massive study, involving more than 200,000 individuals from 53 countries, measured sex drive by asking participants to respond to statements such as “I have a strong sex drive” and “It doesn’t take much to get me sexually excited.”16 In every nation, from Thailand to Croatia to Trinidad, men reported having a higher sex drive than did women. Similarly, large gender differences emerge from studies of masturbation rates and pornography consumption. The sex differences in sex drive prove just as large in nations with high levels of gender equality, such as Sweden and Denmark, as they do in nations with lower levels of gender equality, such as Bangladesh, Egypt, and Nigeria—findings that contradict the notion that these sex differences are caused by economic gender inequality.
Another psychological solution to the problem of gaining sexual access to a variety of partners is to let little time elapse before seeking sexual intercourse. The less time he permits to elapse before obtaining sexual intercourse, the larger the number of women a man can successfully bed. Large time investments absorb more of a man’s mating effort and interfere with solving the problem of number and variety. In the business world, time is money. In the mating world, time is sexual opportunity.
Men and women in the study of short-term and long-term mating rated how likely they would be to consent to sex with someone they viewed as desirable if they had known the person for only an hour, a day, a week, a month, six months, a year, two years, or five years. Both men and women said that they would probably have sex upon knowing an attractive person for five years. At every shorter interval, however, men exceeded women in their reported likelihood of having sex. Five years or six months—it was all the same for men. They expressed equal eagerness for sex with women they had known for either length of time. In contrast, women dropped from probable consent to sex after five years’ acquaintance to neutral feelings about sex after knowing a person for six months.
Having known a potential mate for only one week, men are still on average positive about the possibility of consenting to sex. Women, in sharp contrast, report being unlikely to have sex after knowing someone for just a week. Upon knowing a potential mate for merely one hour, men are slightly disinclined to consider having sex, but the disinclination is not strong. For most women, sex after just one hour is a virtual impossibility. These fundamental sex differences have been extensively documented across the globe, including in Norway, Denmark, and Sweden.17 As with men’s desires, men’s inclination to let little time elapse before seeking sexual intercourse offers a partial solution to the adaptive problem of gaining sexual access to a variety of partners.
Studies of Internet dating point to the same conclusion. Barry Kuhle conducted a study of users of Tinder, a dating site geared heavily toward short-term mating.18 Even though it is widely seen as a site for sexual hookups, men more than women expressed significantly greater interest in immediate short-term sex. Men reported “swiping right” on dozens or even hundreds of female profiles in the hope that a few would reciprocate. Women were considerably more selective, picking just one or a few for potential matches. Male lust, seemingly insatiable, drives men’s search for sexual variety in the modern world of Internet mating.
Standards for Short-Term Mates
Another psychological solution to securing a variety of casual sex partners is men’s relaxation of their standards for acceptable partners. High standards for attributes such as age, intelligence, personality, and marital status function to exclude the majority of potential mates from consideration. Relaxed standards ensure the presence of more eligible players.
College students provided information about the minimum and maximum acceptable ages of a partner for a short-term or long-term sexual relationship. For a brief hookup, college men accepted an age range that was roughly four years wider than women did. Men were willing to have sex in the short run with members of the opposite sex who were as young as sixteen and as old as twenty-eight, whereas women required men to be at least eighteen but no older than twenty-six. This relaxation of age restrictions by men did not apply to committed mating, for which the minimum age was seventeen and the maximum twenty-two. For women the minimum age for committed mating was nineteen and the maximum twenty-five.
Men also relaxed their standards for a wide variety of other characteristics. Out of the sixty-seven characteristics nominated as potentially desirable in a casual mate, men required lower levels of such assets as charm, athleticism, education, generosity, honesty, independence, kindness, intellectuality, loyalty, sense of humor, sociability, wealth, responsibility, spontaneity, cooperativeness, and emotional stability. Men’s relaxation of standards helps to solve the problem of gaining access to a variety of sex partners.
When the college students rated sixty-one undesirable characteristics, women rated roughly one-third of them as more undesirable than men did in the context of casual sex. In this context, men had less objection to drawbacks such as mental abuse, violence, bisexuality, dislike by others, excessive drinking, ignorance, lack of education, possessiveness, promiscuity, selfishness, lack of humor, and lack of sensuality. In contrast, men rated only four negative characteristics as significantly more undesirable than women did: a low sex drive, physical unattractiveness, a need for commitment, and hairiness. Men clearly relaxed their standards more than women did for brief sexual encounters.
Lowered standards, however, are still standards. Indeed, men’s standards for sexual affairs reveal a precise strategy to gain sexual variety. Compared with their long-term preferences, men seeking casual sex disliked women who were prudish or conservative or had a low sex drive. In contrast to their long-term preferences, men valued sexual experience in a potential temporary sex partner, which reflects a belief that sexually experienced women are more sexually accessible to them than women who are sexually inexperienced. Men disliked promiscuity or indiscriminate sexuality in a potential wife or committed mate but believed that promiscuity was either neutral or even mildly desirable in a potential sex partner. Promiscuity, high sex drive, and sexual experience in a woman probably signal an increased likelihood that a man can gain sexual access for the short run. Prudishness and low sex drive, in contrast, signal a difficulty in gaining sexual access and thus interfere with men’s short-term sexual strategy.
One distinguishing feature of men’s relaxation of standards for short-term sex partners involves the need for commitment. In contrast to the tremendous positive value of +2.17 out of 3.00 that men placed on commitment when seeking a marriage partner, men seeking a casual hookup disliked a quest for commitment in a woman, judging it – 1.40, or undesirable, in a short-term partner.19 Furthermore, men were not particularly bothered by a woman’s marital status when they evaluated casual sex partners; a woman’s commitment to another man reduced the odds that she would try to extract a commitment from them. These findings confirm that men shift their desires to minimize their investment in a casual mating, providing an additional clue to an evolutionary history in which men sometimes sought casual, uncommitted sex.
The Coolidge Effect
The story is told that President Calvin Coolidge and his wife, Grace, were being given separate tours of newly formed government farms. Upon passing the chicken coops and noticing a rooster vigorously copulating with a hen, Mrs. Coolidge inquired about how often the rooster performed this duty. “Dozens of times each day,” replied the guide. Mrs. Coolidge asked the guide to “please mention this fact to the president.” When the president passed by later and was informed of the sexual vigor of the rooster, he asked, “Always with the same hen?” “Oh, no,” the guide replied, “a different one each time.” “Please tell that to Mrs. Coolidge,” said the president. And so the “Coolidge effect” was named—the male tendency to be sexually re-aroused upon the presentation of novel females, giving males an impulse to mate with multiple females.
The Coolidge effect is a widespread mammalian trait that has been documented many times.20 Male rats, rams, cattle, and sheep all show the effect. In a typical study, a cow is placed in a bull pen, and after copulation the cow is replaced with another cow. The bull’s sexual response continues unabated with each new cow but diminishes quickly when the same cow is left in the pen. Males continue to become aroused to the point of ejaculation in response to novel females. The response to the eighth, the tenth, or the twelfth female is nearly as strong as the response to the first.
Sexual arousal to novelty occurs despite a variety of attempts to diminish it. For example, when researchers disguised ewes postcopulation with a canvas covering, the rams were never fooled.21 Their response to a female with whom they had already copulated was always lower than with a novel female. The diminished drive was not a result of the female’s having had sex perse; the renewed drive occurred just as often if the novel female had already copulated with another male. And the male remained relatively uninterested if the original female was merely removed and reintroduced.
Men across cultures also show the Coolidge effect. In Western culture, the frequency of intercourse with a long-term partner declines as the relationship lengthens. Compared to the first month of marriage, intercourse is roughly half as frequent after one year of marriage, and it declines more gradually thereafter. As Donald Symons notes, “The waning of lust for one’s wife is adaptive . . . because it promotes a roving eye.”22 Human wanderlust takes many forms. Men in most cultures pursue extramarital sex more often than do their wives. The Kinsey study, for example, found that 50 percent of men but only 26 percent of women had extramarital affairs.23 Some studies show that the gap may be narrowing. One study of 8,000 married men and women found that 40 percent of the men and 36 percent of the women reported at least one affair. The Hite reports on sexuality suggest figures as high as 75 percent for men and 70 percent for women, although these samples are acknowledged not to be representative.24 More representative samples, such as Morton Hunt’s survey of 982 men and 1,044 women, yielded an incidence of 41 percent for men and 18 percent for women.25 Despite these varying estimates, and a possible narrowing of the gap between the sexes, all studies show sex differences in the incidence and frequency of affairs: more men have affairs more often, and with more partners, than women do.26
Spouse swapping, “swinging,” and polyamory are nearly always initiated by husbands, not by wives.27 A Muria man from India summarized the male desire for variety succinctly: “You don’t want to eat the same vegetable every day.”28 A Kgatla man from South Africa described his sexual desires about his two wives: “I find them both equally desirable, but when I have slept with one for three days, by the fourth day she has wearied me, and when I go to the other I find that I have greater passion, she seems more attractive than the first, but it is not really so, for when I return to the latter again there is the same renewed passion.”29
The anthropologist Thomas Gregor describes the sexual feelings of Amazonian Mehinaku men in this way: “Women’s sexual attractiveness varies from ‘flavorless’ (mana) to the ‘delicious’ (awirintya) . . . sad to say, sex with spouses is said to be mana, in contrast with sex with lovers, which is nearly always awirintyapa.”30 Gustav Flaubert writes of Madame Bovary that she is “like any other mistress; and the charm of novelty, gradually slipping away like a garment, laid bare the eternal monotony of passion, whose forms and phrases are forever the same.”31
Studies of men’s sexual arousal to pornography support the existence of the Coolidge effect in humans.32 Using physiological measures of penile tumescence and self-reports of sexual arousal, researchers have shown that men viewing the same erotic images repeatedly experience progressively diminished arousal with each viewing. Over the same time period, men exposed to erotic images of different women became consistently re-aroused. The modern explosion of Internet pornography, now a multibillion-dollar business, owes its success largely to hijacking men’s evolved sexual psychology.
Kinsey sums it up best: “There seems to be no question but that the human male would be promiscuous in his choice of sexual partners throughout the whole of his life if there were no social restrictions. . . . The human female is much less interested in a variety of partners.”33
Sexual Fantasies
Sexual fantasies provide another psychological clue to men’s desire for low-commitment sex. One of several music videos targeted to adolescent men shows a male rock star cavorting across a beach strewn with dozens of beautiful bikini-clad women. Another shows a male rock star caressing the shapely legs of one woman after another as he sings. Yet another shows a rapper gazing at dozens of women who are wearing only underwear. Since these videos are designed to appeal to male audiences, the implication is clear. A prominent male sexual fantasy is to have sexual access to dozens of different young, beautiful women who respond eagerly and willingly.
Men’s and women’s sexual fantasies differ greatly. Studies from Japan, the United Kingdom, and the United States show that men have roughly twice as many sexual fantasies as women.34 In their sleep men are more likely than women to dream about sexual events. Men’s sexual fantasies more often include strangers, multiple partners, and anonymous partners. Most men report that during a single fantasy episode they sometimes change sexual partners; most women rarely change sex partners within one fantasy. In Hunt’s survey, 32 percent of men but only 8 percent of women reported having imagined sexual encounters with over 1,000 different partners in their lifetime. Fantasies about group sex occurred among 33 percent of the men but only 18 percent of the women.35 A typical male fantasy, as one man told another researcher, is having “six or more naked women licking, kissing, and fellating me.”36 Another man reported the fantasy of “being the mayor of a small town filled with nude girls from 20 to 24. I like to take walks, and pick out the best-looking one that day, and she engages in intercourse with me. All the women have sex with me any time I want.”37 Numbers and novelty are key ingredients of men’s fantasy lives.
Male fantasies are heavily visual, focusing on smooth skin and body parts, notably breasts, genitals, buttocks, legs, and mouths. During their sexual fantasies, 81 percent of men but only 43 percent of women focus on visual images rather than feelings. Attractive women with lots of exposed skin who show signs of easy access and no commitment are key components of men’s fantasies. As Bruce Ellis and Donald Symons observe, “The most striking feature of [male fantasy] is that sex is sheer lust and physical gratification, devoid of encumbering relationships, emotional elaboration, complicated plot lines, flirtation, courtship, and extended foreplay.”38 These fantasies reveal a psychology attuned to seeking sexual access to multiple partners.
Women’s sexual fantasies, in contrast, often contain familiar partners. Fifty-nine percent of American women but only 28 percent of American men reported that their sexual fantasies typically focused on someone with whom they were already romantically and sexually involved. Emotions and personality are crucial for women. Forty-one percent of the women but only 16 percent of the men reported that they focused most heavily on the personal and emotional characteristics of the fantasized partner. And 57 percent of women but only 19 percent of men reported that they focused on feelings as opposed to visual images. As one woman observed: “I usually think about the guy I am with. Sometimes I realize that the feelings will overwhelm me, envelop me, sweep me away.”39 Women emphasize tenderness, romance, and personal involvement in their sexual fantasies. They also pay more attention to how their partner responds to them than to visual images of the partner.40
Hooking Up and Sexual Regret
The differing nature of sexual regret in men and women offers further evidence of men’s evolved psychology of short-term mating. Regret is a powerful emotion. We rue the mistakes we have made, a feeling that probably functions to help us make better decisions in the future. Sexual regret occurs in two domains—missed sexual opportunities (sexual omission) and committed sexual actions (sexual commission). In studies of more than 23,000 individuals, men more than women reported regretting missed sexual opportunities.41 These included not having more sex when younger, not having more sex when single, and failing to act on a sexual opportunity with a particularly attractive person. Women were more likely to regret sexual acts of commission, such as losing virginity to the wrong person, hooking up with a person with low mate value when drunk, and having sex with someone who was not interested in a relationship.
Women are more likely than men to experience negative emotions after a hookup. Men are more likely to regret when the woman they hooked up with wants a more serious relationship. Men report that their ideal outcome of a hookup would be more hookups in the future. Women are more likely to report that their ideal outcome would be a romantic relationship. Following hookups, women are more likely than men to report feeling “used” and experiencing depressed mood.42 There are important individual differences within each gender, of course; some women just want casual sex, and some men yearn for deeper connection. The overall gender differences in sexual regret and post-hookup feelings, however, provide additional clues that reveal a fundamental difference in men’s and women’s sexual psychology.
The Closing Time Effect and the Post-Orgasm Shift
Another psychological clue to men’s strategy of casual sex comes from studies that examine shifts in men’s judgments of attractiveness over the course of an evening at a singles bar. In one study, 137 men and 80 women were approached at 9:00 p.m., 10:30, and 12:00 midnight and asked to rate the attractiveness of members of the opposite sex in the bar using a 10-point scale.43 As closing time approached, men viewed women as increasingly attractive. The judgments at 9:00 were 5.5, but by midnight they had increased to over 6.5. Women’s judgments of men’s attractiveness also increased over time. But women’s ratings overall of the male bar patrons were lower than men’s ratings of women. At 9:00, women rated the men at the bar as just below the average of 5.0, which increased near midnight to only 5.5.
Men’s shift in perceptions of women’s attractiveness near closing time occurs regardless of how much alcohol they have consumed. Whether a man has consumed a single drink or six drinks has no effect on the shift in his view of women as more attractive near closing time. The often noted “beer goggles” phenomenon, whereby women presumably are viewed as more attractive with men’s increasing intoxication, may instead be attributable to a psychological mechanism sensitive to decreasing opportunities for sexual success. As the evening progresses and a man has not yet been successful in picking up a woman, he views the remaining women in the bar as increasingly attractive, a shift that should increase his motivation to seek sex from the remaining women in the bar.
Another perceptual shift may take place after men have an orgasm with a casual sex partner with whom they wish no further involvement. Some men report viewing a sex partner as highly attractive before orgasm, but then after orgasm, a mere ten seconds later, viewing her as less attractive or even unappealing. Martie Haselton and I found that these shifts occur primarily among men who are dispositionally inclined to pursue a short-term mating strategy.44 They do not occur for long-term-oriented men, and they do not occur for women regardless of mating strategy. The negative shift in attraction following orgasm may function to prompt a hasty postcopulatory departure to reduce risks to the man such as getting involved in an unwanted commitment.
Sexual Orientation and Casual Sex
A further clue to the significant role of casual mating in men’s sexual repertoire comes from examining the sexual behavior of homosexual men and women. Donald Symons notes that male homosexual sexuality is unconstrained by women’s dictates of romance, involvement, and commitment. Similarly, lesbian sexuality is unconstrained by men’s dictates and demands. The actual behavior of those with a same-sex sexual orientation therefore provides a window for viewing the nature of all men’s and women’s sexual desires, unclouded by the compromises imposed by the sexual strategies of the other sex.
Gay men are much more interested in casual sex with strangers than are lesbian women.45 Whereas male homosexuals often pursue brief encounters, lesbians rarely do. Whereas male homosexuals frequently search for new and varied sex partners, lesbians are more likely to settle into intimate, lasting, committed relationships. One study found that 94 percent of male homosexuals had more than fifteen sex partners, whereas only 15 percent of lesbians had that many.46 The more extensive Kinsey study conducted in San Francisco in the 1980s found that almost one-half of the male homosexuals who participated had over 500 different sex partners, mostly strangers met in baths or bars.47 Some modern dating apps such as Grindr are specifically designed to facilitate these sexual opportunities. When men are unconstrained by the courtship and commitment requirements women often want, they freely satisfy their desires for casual sex with a variety of partners.
In their proclivities for casual sex, as in their long-term mate preferences, homosexual males are similar to heterosexual males and lesbian women are similar to heterosexual women. Homosexual proclivities reveal fundamental differences between men and women in the centrality of casual sex. Symons notes that “heterosexual men would be as likely as homosexual men to have sex most often with strangers, to participate in anonymous orgies in public baths, and to stop off in public rest rooms for five minutes of fellatio on the way home from work if women were interested in these activities. But women are not interested.”48
Prostitution
Men’s desire for casual sex creates a demand for prostitution; many men, including married men, are willing to pay for casual sex.49 Prostitution occurs in nearly every society. In the United States, there are an estimated 1 million active prostitutes, although prostitution is legal only in some counties of the state of Nevada. In Germany, where prostitution is legal, there are roughly 400,000 part-time or full-time prostitutes. Estimates suggest 500,000 prostitutes in Mexico, 800,000 in the Philippines, 3 million in India, and 5 million in China. In all cultures, men are overwhelmingly the consumers. Kinsey found that 69 percent of American men had been to a prostitute, and for 15 percent of them prostitution was a regular sexual outlet. The corresponding numbers for women were so low that they were not even reported.50
The prevalence of prostitution does not imply that it is an adaptation, something targeted by selection. Rather, it can be understood as a consequence of two factors operating simultaneously—men’s desire for low-cost casual sex and women either choosing or being compelled by economic necessity or other factors to offer sexual services for material gain.
The psychological clues that reveal men’s strategies for casual sex are numerous: sexual fantasies, the Coolidge effect, lust, sex drive, inclination to seek intercourse rapidly, relaxation of standards, attitudes toward hookups, emotions of sexual regret, the closing time effect, post-orgasm shifts in judgments of women’s attractiveness, homosexual proclivities, and willingness to use prostitution as a sexual outlet. These psychological clues reveal an evolutionary past that favored men who had short-term mating in their sexual repertoire alongside the pursuit of a long-term mate. But heterosexual men need consenting women for casual sex.
The Hidden Side of Women’s Short-Term Sexuality
The reproductive benefits to men of casual sex are large and direct, but the benefits that women reap from short-term mating were largely neglected until evolutionary psychologists began to investigate them. Although women cannot increase the number of children they bear by having sex with multiple partners, they can gain other important advantages from casual sex as one strategy within a flexible sexual repertoire.51 Ancestral women must have sought casual sex for its benefits in some contexts at some times, because if there had been no willing women, men could not possibly have pursued their own interest in short-term sex.
For ancestral women, unlike men, seeking sex as an end in itself is unlikely to have been a powerful goal of casual mating, for the simple reason that sperm have never been scarce. Access to more sperm would not have increased a woman’s reproductive success. Minimal sexual access is all a woman needs, and there is rarely a shortage of men willing to provide the minimum. Additional sperm are unnecessary for fertilization.
One key benefit of casual sex to women is immediate access to resources. Imagine a food shortage hitting an ancestral tribe thousands of years ago. Game is scarce. The first frost has settled ominously. Bushes no longer yield berries. A lucky hunter takes down a deer. A woman watches him return from the hunt, hunger pangs gnawing. She flirts with him. Although they do not discuss any explicit exchange, her sexual enticements make him more than willing to provide her with a portion of the deer meat. Sex for resources, or resources for sex—the two have been exchanged in millions of transactions over the millennia of human existence.
In many traditional societies, such as the Mehinaku of Amazonia and the natives of the Trobriand Islands, men bring gifts such as food, jewelry, tobacco, betel nuts, turtle shell rings, or armlets to their mistresses. Women deny sex if the gifts stop flowing. A woman might say, “You have no payment to give me—I refuse.”52 A Trobriand man’s reputation among women suffers if he fails to bring gifts, and this interferes with his future ability to attract mistresses. Trobriand women benefit materially through their affairs.
Modern women’s preferences in a lover provide psychological clues to the evolutionary history of the material and economic benefits women gained from brief sexual encounters. Women especially value four characteristics in temporary lovers more than in committed mates—spending a lot of money on them from the beginning, giving them gifts from the beginning, having an extravagant lifestyle, and being generous with their resources.53 Women judge these attributes to be mildly desirable in husbands but quite desirable in casual sex partners. Women dislike frugality and signs of stinginess in a lover; these qualities signal that the man is reluctant to devote an immediate supply of resources. These psychological preferences reveal that securing immediate resources is a key adaptive benefit that women secure through affairs.
The benefit of economic resources from casual sex is most starkly revealed in extreme cases such as prostitution. In cross-cultural perspective, many women who become prostitutes do so out of economic necessity because they lack suitable opportunities for marriage. Women who have been divorced by a man because of adultery, for example, have often become unmarriageable in cultures such as Taiwan Hokkien or the Somalis.54 Chinese and Burmese women may be unmarriageable if they are not virgins. Women among the Aztec and Ifugao are unmarriageable if they have a disease. In all these societies, unmarriageable women sometimes resort to prostitution to gain the economic benefits needed for survival.
Some women, however, say that they turn to prostitution to avoid the drudgery of marriage. Maylay women in Singapore, for example, become prostitutes to avoid the hard work expected of wives, which includes the gathering of firewood and the laundering of clothes. And among the Amhara and Bemba, prostitutes earn enough through casual sex to hire men to do the work that is normally expected of wives. Immediate economic resources, in short, remain a powerful benefit to women who engage in temporary sexual liaisons.
Sexual affairs also provide an opportunity to evaluate potential husbands by supplying additional information that is unavailable through mere dating without sexual intercourse. Given the tremendous reproductive importance of selecting the right husband, women devote great effort to evaluation and assessment. Affairs prior to marriage allow a woman to assess the intentions of the prospective mate—whether he is seeking a brief sexual encounter or a marriage partner, and hence the likelihood that he will abandon her. An affair allows her to evaluate his personality characteristics—how he holds up under stress and how reliable he is. It allows her to see through any deception that might be present—whether he is truly free or already involved in a serious relationship. And it allows her to assess his value as a mate or to learn how attractive he is to other women.
Sexual intercourse before marriage provides important information about the long-term viability of a couple’s relationship by giving them the opportunity to evaluate their sexual compatibility. Through sex women can gauge such qualities as a man’s sensitivity, his concern with her happiness, and his flexibility. Sexually incompatible couples divorce more often and are more likely to experience adultery.55 Twenty-nine percent of men and women questioned by the sex researchers Samuel Janus and Cynthia Janus stated that sexual problems were the primary reason for their divorce—the reason most often mentioned. The potential costs inflicted by an unfaithful mate and by divorce potentially can be avoided by assessing sexual compatibility before making a commitment.
Women’s preferences for short-term mates reveal hints that they use casual sex to evaluate possible marriage partners. If women sought short-term mates simply for opportunistic sex, as many men do, certain characteristics would not be particularly bothersome, such as a man’s preexisting committed relationship or his promiscuity. Women, like men, would find promiscuity in a prospective lover to be neutral or mildly desirable.56 In truth, however, women regard a preexisting relationship or promiscuous tendencies in a prospective lover as highly undesirable, since they signal unavailability as a potentially committed partner or the repeated pursuit of a short-term sexual strategy. These characteristics decrease the woman’s odds of entering a long-term relationship with the man. They convey powerfully that the man cannot remain faithful and is a poor long-term mating prospect. And they interfere with the function of extracting immediate resources, since men who are promiscuous or whose resources are tied up in a serious relationship have fewer unencumbered assets to allocate.
Women’s desires in a short-term sex partner strongly resemble their desires in a husband.57 In both cases, women want someone who is kind, romantic, understanding, exciting, stable, healthy, humorous, and generous with his resources. In both contexts, women desire men who are tall, athletic, and attractive. Men’s preferences, in marked contrast, shift abruptly with the mating context. The relative constancy of women’s preferences in both scenarios supports the hypothesis that some women see casual mates as potential husbands and thus impose high standards for both.
Mate Switching and Backup Mates
Through casual sex, women may also secure added protection against conflicts that arise with other men or with competitors. Having a second mate who will defend and protect her may be especially advantageous for women in societies where they are at considerable risk of attack or rape. In some societies, such as the Yanomamö of Venezuela, women are vulnerable to male violence, including physical abuse, rape, and even the killing of their children, when they lack the protection of a mate.58 This vulnerability is illustrated by the account of a Brazilian woman who was kidnapped by Yanomamö men.59 When men from another village tried to rape her, not a single Yanomamö man came to her defense because she was not married to any of them and had no special male friends to protect her.
The use of such special friendships for protection has a primate precedent among savanna baboons.60 Female baboons form special friendships with one or more males other than their primary mates, and these friends protect them against sexual harassment from other males. Females show a marked preference for mating with their friends when they enter estrus, suggesting a strategy of exchanging sex for protection. As Robert Smith points out:
A primary mate cannot always be available to defend his wife and children and, in his absence, it may be advantageous for a female to consort with another male for the protection he may offer. . . . Absence of the primary mate [for example, when he is off hunting] may create the opportunity and need for extrabond mating. . . . A male may be inclined to protect the children of a married lover on the chance that his genes are represented among them.61
A lover may also serve as a potential replacement for a woman’s regular mate if he should desert, become ill or injured, prove to be infertile, or die, which were not unusual events in ancestral environments. Her regular mate might fail to return from the hunt, for example, or be killed in a tribal war. Men’s status might change over time—for instance, a woman married to a head man who is deposed, whose position is usurped, and whose resources are co-opted might benefit by positioning herself to replace him quickly, without having to start over again. A woman who must delay replacing her mate by starting over is forced to incur the costs of a new search for a mate while her own desirability declines. Women benefit from having other men as potential backup mates.
The mate-switching function has been observed in the spotted sandpiper (Actitis macularia), a polyandrous shorebird studied on Little Pelican Island in Leech Lake, Minnesota. The biologists Mark Colwell and Lewis Oring, through 4,000 hours of field observation, discovered that a female spotted sandpiper who engages in extra-pair copulations with another male has an increased likelihood of becoming an enduring mate with that male in the future.62 The females use the copulation as a way to test the receptivity and availability of the male. Male spotted sandpipers, however, sometimes foil these attempts at mate switching. Some males move several territories away from their home base when seeking extra-pair copulations, apparently so that the female will not detect that they are already mated. Despite this conflict between the sexes, the fact that the adulterers often end up as committed mates suggests that the extra-pair matings function as a means to switch mates.
Evidence for the mate-switching function of casual sex in humans comes from several sources. Women tend to have affairs when they are unhappy with their primary relationship, whereas men who have affairs are no less happy with their marriages than men who refrain.63 Heidi Greiling and I conducted the second study, which revealed that women sometimes have affairs when they are trying to replace their current mate or in order to make it easier to break off with a current mate.64
Casual sex partners sometimes bestow elevated status on their temporary mates. Women sometimes elevate their status by mating with a prestigious man, even if it is just an affair. In the economics of the mating marketplace, people assume that the woman must be special, since prestigious men generally have their pick of the most desirable women. Women may gain access to a higher social stratum, from which they might secure a permanent mate. Women also can elevate their status within their own social circles and potentially secure a more desirable husband.
Possible Genetic Benefits of Short-Term Sex
It is also possible for women to gain superior genes through casual sex that are passed on to their children. Given men’s proclivities for opportunistic sex, the economics of the mating marketplace render it far easier for a woman to get a man from a higher stratum or with better genes to have sex with her than it is for her to get him to marry her. A woman might try to secure the investment of a lower-ranking man by marrying him, for example, while simultaneously securing the genes of a higher-ranking man by mating with him casually. This dual strategy apparently exists in Great Britain, where the biologists Robin Baker and Mark Bellis have discovered that women typically have affairs with men who are higher in status than their husbands.65
One version of the better genes theory has been called the “sexy son hypothesis.”66 According to this view, women prefer to have casual sex with men who are attractive to other women because they will have sons who possess the same charming characteristics. Women in the next generation will therefore find these sons attractive, and the sons will enjoy greater mating success than the sons of women who mate with men who are not regarded as attractive by most women.
Evidence for this theory comes from a study that identified key exceptions to women’s more stringent selection criteria for long-term partners. Women are more exacting with regard to physical attractiveness in a casual encounter than they are in a permanent mate.67 The preference for physically attractive casual sex partners may be a psychological clue to a human evolutionary history in which women benefited through the success of their sexy sons.
The strongest evidence of the good genes hypothesis comes from studies of women’s preference shifts when ovulating. Large-scale statistical reviews of the three dozen or so studies of cycle shifts find that ovulating women not on hormonal contraceptives do indeed show stronger preferences for masculine men, especially for masculine bodies and voices, as well as symmetrical men.68 Only women with mates low in genetic quality, however, stand to reap genetic benefits from an affair with a man who has the hypothesized good-genes indicators. And some researchers argue that other hypotheses better explain ovulation preference shifts. Ovulating women, for example, experience a higher sex drive, may feel sexier and more desirable, and so feel that they can successfully attract men of higher mate value.69 The next decade of research will undoubtedly delve more deeply into the hidden genetic benefits to women of pursuing a short-term sexual strategy.
One type of mating system puts sharp constraints on women’s short-term mating and any benefits that might accrue from them—arranged marriages. Although we can never know for sure, anthropologists believe that many women during human evolutionary history did not contract their own marriages; the evidence for this belief is the prevalence of marriages arranged by fathers and other kin in today’s tribal cultures, which are assumed to resemble the conditions under which humans evolved.70 The practice of arranged marriage is still common in many other parts of the world as well, such as India, Kenya, and the Middle East. Arranged marriages restrict the opportunities for women to reap the benefits of short-term mating. Even where matings are arranged by parents and kin, however, women often exert some influence over their sexual and marital decisions by manipulating their parents, carrying on clandestine affairs, defying their parents’ wishes, and sometimes eloping. These forms of personal choice open the window to the benefits for women of short-term mating, including genetic benefits, even when their marriage is arranged by others.
Costs of Casual Sex
All sexual strategies carry costs, and casual sex is no exception. Men risk contracting sexually transmitted diseases, acquiring a reputation as a womanizer or “man-whore,” or suffering injury from a jealous husband. A significant proportion of murders across cultures occur because jealous men suspect their mates of infidelity.71 Unfaithful married men risk retaliatory affairs by their wives and costly divorces. Short-term sexual strategies also take time, energy, and economic resources.
Women sometimes incur more severe costs than men do. Women risk impairing their desirability if they develop a reputation for promiscuity, since men prize fidelity in a potential long-term mate. Women known as promiscuous suffer reputational damage even in relatively promiscuous cultures, such as among the Swedes and the Ache Indians.72 A woman who adopts an exclusively short-term sexual strategy is at greater risk of physical and sexual abuse. Although women in marriages are also subjected to battering and even rape by husbands, the alarming statistics on the incidence of date rape, which some estimate to be 15 or 20 percent of all college women, reveal that women who are not in long-term relationships are at considerable risk.73 Mate preferences, if judiciously applied to avoid potentially dangerous men, can minimize these risks.
Unmarried women who pursue casual sex risk getting pregnant and bearing a child without the benefits of an investing partner. In ancestral times, these children would have been at much greater risk of disease, injury, and death.74 Some women commit infanticide in the absence of an investing man. In Canada, for example, single women delivered only 12 percent of the babies born between 1977 and 1983, but they committed just over 50 percent of the sixty-four maternal infanticides reported to the police.75 This trend occurs across cultures as well, such as among the Baganda of Africa. But even this solution does not cancel the substantial other costs that women incur: nine months of gestation, reputational damage, and lost mating opportunities.
An unfaithful married woman risks the withdrawal of resources by her husband. From a reproductive perspective, she may be wasting valuable time in an extramarital liaison, obtaining sperm that are unnecessary for reproduction.76 Furthermore, she risks increasing the sibling competition among her children, who may have weaker ties because they were fathered by different men.77
Short-term mating, in short, poses hazards for both sexes. But because there are powerful benefits as well, women and men have evolved psychological mechanisms to select circumstances in which the costs of short-term mating are minimized and the benefits increased.
Circumstances Favorable for Casual Sex
Everyone knows some men who are womanizers and others who are exclusively monogamous. Everyone knows some women who enjoy casual sex and others who would not dream of sex without commitment. Individuals differ in their proclivities for casual mating. Individuals also shift their proclivities at different times and in different contexts. These variations in sexual strategy depend on a range of personal, social, cultural, and ecological conditions.
The absence of an investing father during childhood has been reliably linked to the pursuit of a short-term sexual strategy. Among the Mayan of Belize and the Ache of Paraguay, for example, father absence has been correlated with men stating that they are unwilling to commit the time, energy, and resources needed to sustain a long-term mateship.78 Women and men who grow up in father-absent homes are likely to reach puberty sooner, to start having intercourse earlier, and to pursue a short-term strategy after their sexual debut.79 Their father’s absence may lead women to infer that men are not reliable investors and thus to pursue a short-term sexual strategy of obtaining some benefits from multiple men rather than trying to secure the sustained investment of one man.
There is some scientific controversy over the direction of effects and precisely what causes individual differences in sexual strategy. Is the shift to a short-term strategy when one’s father is absent due to the harsher or more unpredictable environment resulting from the father’s lack of investment? Or is it due to genes that father-absent dads pass on to their children? Yet another possibility is that stepfather presence, rather than father absence, prompts kids to spring into sexual action sooner. Moreover, biological fathers engage in heavier “daughter guarding,” which prevents girls from acting on their sexual impulses and encourages a long-term mating strategy. Future research is required to sort out these causal alternatives, but it is clear that individuals differ markedly in their inclination for short-term sex.
Casual sex is also related to people’s developmental stage in life. Adolescents in many cultures are more likely to use temporary mating as a means of assessing their value on the mating market, experimenting with different strategies, honing their attraction skills, and clarifying their own preferences. After they have done so, they are often ready for commitment or marriage. The fact that premarital adolescent sexual experimentation is tolerated and even encouraged in some cultures, such as the Mehinaku of Amazonia, provides a clue that short-term mating is related to one’s stage in life.80
The transitions between committed matings offer additional opportunities for casual sex. Upon divorce, for example, it is crucial to reassess one’s value on the current mating market. The presence of children from the marriage generally lowers the desirability of divorced people. The elevated status that comes with being more advanced in their career, conversely, may raise their desirability. Precisely how all these changed circumstances affect a particular person may be evaluated by brief encounters, which allow a person to gauge more precisely his or her desirability on the mating market.
The abundance or shortage of eligible men relative to eligible women is another critical context for temporary mating. Many factors affect this sex ratio, including wars, which kill larger numbers of men than women; risky activities such as fights, which more frequently affect men; intentional homicides, in which roughly seven times more men than women die; and differential remarriage rates by age, whereby women remarry less and less often than men with increasing age. The modern phenomenon of more women than men enrolling in many colleges and universities in North America and western Europe creates a sex ratio imbalance among the college-educated mating pool. At the University of Texas at Austin, for example, the student body in 2016 was 54 percent women and 46 percent men. This may not sound like a huge disparity, but in fact it translates into a surplus of 17 percent more women than men. China’s historical one-child policy, in contrast, created a surplus of men because parents selectively aborted female fetuses.
Men shift to short-term sexual strategies when many women are sexually available because the sex ratio is in their favor and they are better able to implement their desire for variety. Among the Ache, for example, men are highly promiscuous because there are 50 percent more women than men. Women shift to casual sex when there is a shortage of investing men available for marriage or when there are few benefits to marriage.81 In some subcultures, notably in areas of concentrated poverty, men often lack the resources that women desire in a permanent mate. Where men do not have resources, women have less reason to mate with only one man. Similarly, when women receive more resources from their kin than from their husbands, they are more likely to engage in extramarital sex.82 Women in these contexts sometimes mate opportunistically with different men, securing greater benefits for themselves and their children. The rise of the hookup culture on many modern college campuses has been partly created by the increasing sex ratio imbalance as higher percentages of women compared to men seek higher education.
In cultures where food is shared communally, women have less incentive to marry and often shift to temporary sex partners. The Ache of Paraguay, for example, communally share food secured from large game hunting. Good hunters do not get a larger share of meat than poor hunters. Women receive the same allotment of food, regardless of whether they have a husband and regardless of the hunting skill of their husband. Hence, there is less incentive for Ache women to remain mated with one man, and about 75 percent of them favor short-term relationships.83 The socialist welfare system of Sweden provides another example. Since food and other material resources are provided to everyone, women have less incentive to marry. As a result, only half of all Swedish couples who live together get married, and members of both sexes often pursue more casual relationships.84
Another factor that is likely to foster brief sexual encounters—although differently for men and women—is one’s future desirability as a mate. A man at the apprenticeship stage of a promising career may pursue only brief affairs, figuring that he will be able to attract a more desirable long-term mate later on, when his career is closer to its peak. A woman whose current desirability is low may reason that she cannot attract a husband of the quality she desires and so may pursue carefree short-term relationships as an alternative to settling for someone who does not meet her standards.
Certain legal, social, and cultural sanctions encourage short-term mating. Roman kings, for example, were permitted to take hundreds of concubines, and like Moroccan emperor Moulay Ismail the Bloodthirsty (see Chapter 3), they cycled the young women out of the harem by the time they reached the age of thirty.85 In Spain and France, it is an accepted cultural tradition that men who can afford it keep mistresses in apartments, a short-term arrangement outside the bounds and bonds of marriage. The ideologies of some isolated groups and communes—a living arrangement that was especially popular during the late 1960s and early 1970s and is present today among those practicing polyamory—encourage sexual experimentation with short-term relationships.
The sexual strategies pursued by other people affect the likelihood of casual sex. When many men pursue short-term relationships, as in Russia in many large-population cities, women are effectively forced into casual mating because fewer men are willing to commit, although some women choose to opt out of the mating game entirely. Or when one spouse has an extramarital affair, then the other may be motivated to even the score. Casual sex is never pursued in a vacuum. It is influenced by stage of life, mate value, sex ratios, cultural traditions, legal sanctions, and the strategies pursued by others. All of these circumstances affect the likelihood that a person will choose casual sex from the menu of human mating strategies.
Casual Sex as a Source of Power
Historically, the scientific study of mating has focused nearly exclusively on marriage. Human anatomy, physiology, psychology, and behavior, however, betray an ancestral past filled with opportunistic sex and affairs. The obvious reproductive advantages of such affairs to men may have blinded scientists to the benefits they had for women as well. Affairs involve willing women. Willing women seek or require benefits.
This picture of human nature may be disturbing to some. Women may not be comforted by the ease with which men are sometimes willing to jump into bed with near-strangers. Men may not be comforted by the knowledge that their partners continue to scan the mating terrain, encourage other men by flirting, offer hints of sexual accessibility, cultivate backup mates, and sometimes cheat with impunity. Human nature can be alarming.
But viewed from another perspective, our possession of a complex repertoire of mating strategies gives us far more power, far more flexibility, and far more control of our own destiny. We choose from a large mating menu and are not doomed to a single, invariant strategy. We tailor our mating strategies to our circumstances, be they an environment characterized by a sex ratio imbalance or a return entry into the mating market after a breakup. Moreover, the rapid rise of Internet dating sites and mobile apps greatly expands the pool of potential mates, providing access to thousands of potential partners we would never otherwise encounter in our day-to-day lives.
Modern technology and contemporary living conditions also allow people to escape many of the costs of casual sex that our ancestors experienced. Effective birth control, for example, allows us to avoid the costs of an unwanted or ill-timed pregnancy. The relative anonymity of urban living diminishes the reputational damage incurred by casual sex. Geographic mobility lowers the often restrictive influence of parents on the mating decisions of their children. And government safety nets lower the risks to survival for children produced by short-term liaisons. These reduced costs foster a fuller expression of the range of human mating within our complex repertoire.
Acknowledging the full diversity of our mating strategies may violate our socialized conceptions of one-and-only bliss. But simultaneously, this knowledge gives us greater power to design our own mating destiny than any humans in our evolutionary past ever possessed.