Casual Sex

Some individuals are more open to affairs or short-term liaisons than others:

“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.”

Sociosexual tendencies are methodologically difficult to study:

“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.”

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.”

Both in ancestral environments and today, humans cannot be characterized as an exclusively pair-bonding species.

“Another clue to the evolutionary existence of casual mating comes from variations in sperm production and insemination. 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.”

This observation echoes the often-voiced claim that spending less than all of one’s time together may increase tension and passion within a romantic relationship.

“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.

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.”

Taken together, these findings suggest that men who are able to sexually satisfy their partner may have a higher likelihood of successful insemination.

“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.”

In this sense, female orgasm can be understood as a potential mate choice mechanism, biasing fertilization toward partners of higher genetic quality.

Lust

“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.”

From an evolutionary viewpoint, male lust functions as a drive toward sexual variety, which would historically have increased the likelihood of impregnating multiple partners.

“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.”

In a short-term mating context, men are often looking for the easy lay.

“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.”

In short-term mating, men apply a different filter: not who would make a good partner, but who is most sexually available.

“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.”

Sexual Fantasies

“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. 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. A typical male fantasy, as one man told another researcher, is having “six or more naked women licking, kissing, and fellating me.” 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.”

There are clear sex differences in both the volume and content of sexual fantasies.

“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.” These fantasies reveal a psychology attuned to seeking sexual access to multiple partners.”

In contrast to this, many female sexual fantasies tend to center on a familiar partner, often a long-term mate, and place greater emphasis on emotional connection and relational context:

“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.” 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.”

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. 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.”

Men regret not acting, women regret acting.

“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. … 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

“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.”

In this context, men can be understood as opportunistic maters, adjusting their preferences as opportunities diminish.

“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. 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.”

Prostitution

The scale of prostitution across societies is remarkable:

“Men’s desire for casual sex creates a demand for prostitution; many men, including married men, are willing to pay for casual sex. 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.”

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. 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.”

So women must have had different reasons than men, in ancestral times, for seeking or being open to short-term sex.

“One key benefit of casual sex to women is immediate access to resources.”

“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. 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.”

Beyond resources, affairs can function as a way for women to evaluate potential long-term mates under real conditions:

“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.”

Affairs also allow women to test sexual compatibility.

“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.”

So beyond resources, women’s short-term mating tendencies seem to function mainly as a way to evaluate long-term mate potential:

“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. 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.”

In a nutshell:

“Women’s desires in a short-term sex partner strongly resemble their desires in a husband. 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

“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.”

Affairs tend to serve different functions for men and women:

“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. 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.”

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. 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.”

Beyond the risk of STDs, a major cost for women is that men generally do not want promiscuity in a long-term partner.

Circumstances Favorable for Casual Sex

“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.”

Short-term relationships can be used to gauge one’s current standing in the mating hierarchy.

“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.”

Like the trajectory of a stock, one’s value on the mating market can dip temporarily—and in those phases it can make sense to wait for better times before committing to a long-term partner.

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.”

For both men and women, it can be unsettling to get a clear view of the other sex’s mating psychology:

“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.”

In modern societies, the relative anonymity of urban living reduces the reputational costs of casual sex.

To cut straight to the chapter’s conclusion:

“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.”

The implication is that a nuanced understanding of human sexual nature allows individuals to recognize and flexibly navigate different mating strategies as circumstances change.