What about the sex difference in sex? In a short chapter entitled “The Sexual Superiority of the Female,” Montagu wrote that while “social conditioning plays a considerable role . . . there is a profound biological difference between the sexes. . . . The male seems to be in a chronic state of sexual irritation. The woman who in a letter to Kinsey described the race of males as ‘a herd of prancing leering goats’ was not far from the truth.” Montagu understood what makes women prodigiously sexual in their own way: the extreme innervation and sensitivity of the clitoris and its short refractory period enable multiple and prolonged orgasms—men are no match for women there—and extend sexual function late in life. But, he believed, women’s arousal is not continuous and impersonal; it is framed in relationships and works best when a partner, male or female, makes an investment that comes from caring.

Everything we have learned in the decade and a half since Montagu’s last edition confirms these differences. Very few men tell women what they are thinking, and this may be a good thing. Women who “get it” may sympathize (up to a point) or not, but either way they are better off than those who believe men are just like them. Even in post-industrial cultures prominent men often display their intentions by taking trophy wives, using supermodels as “elbow candy,” and having illicit sex with much younger women, even in the public eye. But only occasionally do they say something revealing.

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Henry Kissinger, Nixon’s national security adviser, was believed to have more power than the secretary of state when a January 19, 1971, article in the New York Times noted that “as a 47-year-old divorcé, he makes society news by squiring such glamour girls as Gloria Steinem in New York, Joanna Barnes and Jill St. John in Hollywood, and Barbara Howar in Washington. Power, he has observed, is the great aphrodisiac.” The article depicts him as insecure despite his power; even at his peak he was a short, chubby, plain-looking man. But this did not interfere with his associations with much younger, very gifted, sought-after, beautiful women, then known without irony as “glamour girls.”

Bill Ackman, currently in his late forties, is a handsome billionaire hedge fund manager married to a beautiful woman his own age, with three children, and not known for any untoward behavior with women. Perhaps that’s why he could be frank in answering his own question in a lecture at the Wharton business school a few years ago. “What motivates people to succeed?” He paused. “Sex. People don’t like to admit it but it’s the primal driver. Fundamentally, what drives most human behavior is basically foreplay.” On CNBC a year or so later, anchor Becky Quick, who had read the quote, offered Ackman a chance to explain himself. He replied, “I was just thinking, you know, well, ultimately we’re animals, right? Motivated by basic . . . you know, why are people motivated to succeed? I mean, just think about guys you went to college with.”

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If that was true of the guys they went to college with, you don’t even want to think about the guys in college today. In the summer of 2013, the New York Times ran a story with a provocative photo of a reclining woman’s bare thigh and the headline “Sex on Campus: She Can Play That Game, Too.” Based on months of interviews and authored by Kate Taylor, it was one of many recent writings suggesting that women want casual sex and multiple partners as much as men do.

The article was about the sex scene at an elite university. In a year of impressive work, Taylor had interviewed sixty young women. She wrote about them generally and in a few cases in particular. I read the article to the end and could only guess that whoever wrote the title hadn’t. It began with a young woman texting her usual hookup guy to ask what he was doing; she ended up having sex with him. “We don’t really like each other in person, sober,” she said, adding that they couldn’t even sit down and have coffee. She said she was too busy succeeding in school and building a high-powered career to have a relationship and emphasized that she didn’t regret any of her one-night stands, describing herself as a true feminist and a strong woman who knows what she wants. She withheld her name, but she still didn’t want the number of men she had slept with printed.

Same game? Another young woman found as a first-year student that nobody seemed to have boyfriends but felt, as she put it, “I can’t just lose my V-card to some random guy.” In the spring she picked a boy she had been dancing with and awarded her card to him. “I’m like, ‘O.K., I could do this now. He’s superhot, I like him, he’s nice.’” Her expectations were very low. But because he let her spend the night and walked her home in the morning, all her friends were “super envious”; she came back with a huge smile on her face because she’d had “such a great first experience.” She stayed on good terms with the boy and during spring break had casual sex with someone else. But she believes boys control the hookup culture: “It’s kind of like a spiral.” She explained that the girls stop anticipating that they will get a boyfriend, “but at the same time, they want to, like, have contact with guys. . . . [They] try not to get attached.”

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Same game? Haley, a senior, reminisced about going to frat parties as a freshman: she’d go in and they’d take her down to a dark basement. “There’s girls dancing in the middle, and there’s guys lurking on the sides and then coming and basically pressing their genitals up against you and trying to dance.” These are dance-floor makeouts, or difmos. After one, she knew she was drunk and asked the boy to take her home; he took her to his room instead and had sex with her semiconscious body. She recalled another boy popping his head into the room: “Yo, did you score?” Eventually she looked back on this episode as rape.

Another girl said she usually ended up giving the boy oral sex, because by the time she got to his room she was sobering up and saw that as her best way out. Another, Kristy, told of making out with a boy in his house when he said, “‘Get down on your knees.’ . . . I was really taken aback, because I was like, no one has ever said that to me before.” The boy said it was fair and, when she balked, he pushed her down. After that, she thought, “‘I’ll just do it.’ . . . I was like, ‘It will be over soon enough.’” Catherine, a senior, looked back on her hookups as a continual source of heartbreak.

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I know those boys. I teach them. I have three daughters who had to navigate these rocky shoals in three different colleges, and they say they were able to keep the hookup culture at a distance. I can’t really be sure, of course, but I do know that many do not. The young women in Taylor’s story are not even remotely playing the same game, and if they think they are, they are way out of their league.

*

The research on this is clear, and transnational. A 2008 study by Anne Campbell was called “The Morning After the Night Before.” A British television station surveyed thousands of people through its website; 998 of the men and 745 of the women who responded were heterosexual and had had a one-night stand. They were asked about their agreement with positive and negative statements about the event. Men were much more likely to report greater sexual satisfaction, wellbeing, and self-confidence, while women were much more likely to feel that they had been used and had let themselves down. Overall, subtracting negative scores from positive ones, men had more than double the net gain from the experience. As for regret, 23 percent of men but 58 percent of women said they would not repeat it.

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Campbell writes of women’s post-tryst distress, “The men had subsequently behaved disrespectfully and dismissively. . . . While not wanting a longer relationship, many women felt a strong sense of rejection”; they said they’d been “blanked.” Women’s positive remarks had to do with being made to feel sexy and wanted, craving male attention, or satisfying curiosity. One said, “I have a very poor self image and the man I slept with was a conquest. . . . He was very popular with other women and very good-looking. I thought that if I slept with him it would put me on a par with my prettier and more worthy peers. Unfortunately it didn’t work and my self esteem/confidence suffered.”

As for the sex itself, men spoke of “euphoria,” “excitement and lust,” and “blowing off sexual steam.” Some women had fun and felt free, but most said things like

• “Thought it would be one of life’s experiences, but it was nothing like the sex found in movies.”

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• “The expectation was better than the reality, the sex was rubbish.”

• “The sex is never particularly satisfying because it is difficult to let go with someone you don’t even know.”

• “Not as good as sex with a partner; they are more into your needs and know your body a lot better.”

Concerns like Is sex all the person was after? and Will the person call or will they dump me? were expressed by 81 percent of women and 17 percent of men. I can almost hear the men asking, Dump you from what?

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Is there something about Britain or Campbell’s methods that produced these results? Many U.S. studies confirm them with other approaches. Anthropologist John Townsend and his colleagues have studied hookups at Syracuse University for over a decade. In 2011 they reported on 335 male and 365 female students. Men were much more likely to endorse casual sex and women to feel a need for attachment and emotional involvement, but men were only somewhat more likely to have had such hookups. This suggests that men are persuading women to have more casual sex than they want while women prevent men from having as much as they want.

In 2012, Justin Garcia of the Kinsey Institute and his colleagues reviewed many studies of hookup culture, including patterns like NSA (no strings attached) sex and FWBs (“friends with benefits,” a.k.a. “hookup buddies” or “fuck buddies”), which provide repeat encounters (“booty calls”) with one person. In one study of 832 college students, 50 percent of men and 26 percent of women had positive emotional reactions after hookups, while for negative reactions the percentages were almost exactly reversed. In another, an online survey of more than twelve thousand students from seventeen colleges, 55 percent of first hookups involved oral sex only for the man and 19 percent only for the woman; 31 percent of men and 10 percent of women had orgasms. In a third study of 761 women, more than half reported at least one experience of unwanted sex. About two-thirds of hookups with vaginal intercourse involved condoms; when only oral sex was involved, the percentage was close to zero.

In “Bare Market: Campus Sex Ratios, Romantic Relationships, and Sexual Behavior,” sociologists Jeremy Uecker and Mark Regnerus studied a representative sample of nearly a thousand single, heterosexual women in four-year coed colleges. The percentage of women on campus predicted attitudes and behavior toward the men on the same campus. Where women were plentiful, they were more likely to say that men were untrustworthy and uninterested in commitment. They expected less from men and found it harder to meet the right kinds of men. They were less likely to have gone on traditional dates, to have had a boyfriend, or to be a virgin and more likely to have had sex in the last month, especially if they didn’t have a boyfriend.

Little wonder that women on those campuses say they don’t want their percentage to go up any further. By the way, if women wanted casual sex as much as men, wouldn’t they be more likely to have it where they are in a minority? They don’t, but where they predominate the market leads them to give up more than they otherwise would. They are competing for men, and the men have more power. Since women now make up about 60 percent of college students and rising, it seems likely that hookups will rise as well. The National Longitudinal Study of Adolescent Health has found a similar effect of sex ratio in high schools, which suggests that the next cohort of college women will be playing the hookup game as they continue to try to please scarce men.

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I recognize that many women speak well of these experiences and often seek them. Women like sex, and some like casual sex. Among other things, they are protecting themselves from relationships that might distract them from their studies and career pursuits. Celibacy is often not a good choice, especially given peer pressure and boy pressure, not to mention desire. If you want to avoid being seen as a child or a prude by women and you like men enough to want to be on good terms with them, hookups can be stimulating and convenient. One time in ten or so, you may even have an orgasm. A so-called fuck buddy can be a reasonable compromise between loneliness and a complicated romantic involvement. But these encounters and relationships are sexually asymmetrical. Someone is often being used, and it is seldom the boy.

*

Recall that Nisa, the !Kung woman who was the subject of my late wife Marjorie Shostak’s classic, said, “Women possess something very important, something that enables men to live: their genitals.” Or as anthropologist Donald Symons, whose pioneering book The Evolution of Human Sexuality helped start this field of research, said, “Among all peoples it is primarily men who court, woo, proposition, seduce, employ love charms and love magic, give gifts in exchange for sex, and use the services of prostitutes. And only men rape. Everywhere sex is understood to be something females have that males want.” This is an exaggeration but one with a great measure of truth. Women forgot this truth when men convinced them that both sexes have the same interests; this was the sexual revolution of the sixties, which involved a lot more change and disappointment for women than for men, at least in the realm of sex. In view of persistent myths about this in the ongoing sexual revolution today, let’s look at the evidence for Symons’s pointed claim.

A 2001 overview in the Personality and Social Psychology Review by Roy Baumeister, Kathleen Catanese, and Kathleen Vohs combed more than 150 studies to answer the question “Is there a gender difference in sex drive?” Overall in these studies,

Men have been shown to have more frequent and more intense sexual desires than women, as reflected in spontaneous thoughts about sex, frequency and variety of sexual fantasies, desired frequency of intercourse, desired number of partners, masturbation, liking for various sexual practices, willingness to forego sex, initiating versus refusing sex, making sacrifices for sex, and other measures.

There were no studies with contrary findings—not a single one indicating stronger sexual motivation in women than men. In one typical study, 90 percent of men but only half of women felt sexual desire at least a few times a week. In another, the average young man was sexually aroused several times a day, the typical young woman “a couple of times a week.” In an Australian survey, people who were in a committed relationship, wanted to have sex, but were not having it were almost exclusively male.

Compared to women, men begin to have sexual intercourse earlier in life (despite later puberty), are less willing to give up sex for any part of life, are more permissive and favorable toward sex, initiate sex much more often in longer relationships, and show more preference for every sexual practice, including, rather astoundingly, cunnilingus. Although men often have physical problems like premature ejaculation and erectile dysfunction, hypoactive sexual desire, whether by diagnosis or self-report, is overwhelmingly female.

In their classic 1989 study, “Gender Differences in Receptivity to Sexual Offers,” social psychologists Russell Clark and Elaine Hatfield had confederates who were college men and women of average attractiveness approach strange but attractive members of the opposite sex on campus and say, “I’ve been noticing you around campus lately and find you very attractive. Would you like to go to bed with me tonight?” Of the men, 75 percent said yes; of the women, 100 percent said no. Some men said things like “Why wait till tonight?” Some women said things that I won’t repeat here.

In 2011, psychologist Terri Conley claimed to have repeated this study; she found that under some conditions, the difference between the sexes was much smaller. However, there was a minor problem with this “replication”: it was purely a paper-and-pencil study. I am sorry, but that is not remotely a repeat of the classic study. Yet the media seized on it as proof that sexual mores have dramatically changed and that women are almost as interested in casual sex as men. The current wish to deny the facts of life is very great.

Suppose we ask what happens when you remove the slight complication of having to deal with another person, whether a stranger, intimate long-term partner, or anything in between. Sex differences in masturbation are consistent and large. Women are much more likely to have never masturbated; women who do masturbate do it much less frequently at all ages than men. In a 2011 summary of meta-analyses and large data sets, psychologists Jennifer Petersen and Janet Shibley Hyde confirmed substantial sex differences in masturbation—even with such blunt (and indeed almost ridiculous, if you are looking at sex differences) measures as whether someone has masturbated in the past year—and in pornography use, in the usual direction.

Consider people’s fantasy lives. Here, too, there is no other person present. You can dream up whatever you want, no risk, no compromise, no complications. In one study, Bruce Ellis and Donald Symons gave three hundred male and female students an anonymous questionnaire. Men (32 percent) were four times as likely as women to say they had fantasized about having sex with more than one thousand different people (by college age). Men were much more likely to say that visual images were more important than touching in their fantasies (66 percent versus 39 percent), women twice as likely to say touching (55 percent versus 28 percent). Men were about twice as likely to focus on visual images rather than feelings, women three times as likely to say feelings. And men were almost three times as likely (48 percent versus 17 percent) to agree that in their fantasies, “the situation quickly includes explicitly sexual activity.” These findings have been repeated in many studies.

Of course, sometimes men want men, and women, women, and these relationships are most instructive. Lesbian relationships, compared to those between gay males, are less sexual at every stage by almost every measure; the phrase “lesbian bed death” may be an exaggeration, but we don’t often hear it said about gay men. In many ways—frequency of intercourse, open relationships, sexually transmitted diseases, and the use of sadomasochistic elements in sex—heterosexual couples are intermediate between lesbian and gay male pairings.

These are old findings, and new research holds few surprises. A 2013 study by sociologist Bethany Everett, as well as research in 2010 by epidemiologist Fujie Xu and colleagues, showed that while bisexual people are at the highest risk, the number of lifetime partners and the amount of sexually transmitted disease are higher in exclusively gay than in heterosexual men and lower in exclusively lesbian than in heterosexual women. So when you remove such complex issues as male dominance and women’s oppression or the desire to please in heterosexual relationships, where the two people involved are of the same sex, male-female differences are larger, not smaller.

Who pays cash for sex? Almost exclusively men. An estimated one-tenth to one-sixth of U.S. men have paid for sex, half of those while they were involved in other relationships. A few women pay for sex, but it’s not just a simple transaction. Women do not buy much pornography or leer at pictures of naked men. Porn customers are overwhelmingly male, and the main counterpart for women is romance fiction. In her insightful 2012 article “The Pop Culture of Sex: An Evolutionary Window on the Worlds of Pornography and Romance,” psychologist Catherine Salmon says, “Romance and pornography are both multibillion dollar industries, and their stark contrasts reflect the deep divide at the heart of male and female erotic fantasies.”

After an extended analysis, she concludes, “Pornography is a male fantasy world of short-term mating success while the romance is a female fantasy world of long-term mating success. At their hearts, that is what they are, fantasies that are reflections of the different ancestral problems faced by males and females in the mating domain.” Even the huge hit novel Fifty Shades of Grey, sympathetically reviewed by some intelligent women and read by millions, is a romance novel that includes soft-core pornography. It depicts a rich and powerful man deeply in love with a younger woman and very concerned about her welfare and her sexual pleasure. The bondage and discipline he subjects her to (because of his self-described mental illness and after elaborately and solicitously gaining her consent) is mild compared to that in male-directed pornography; she is depicted by the (female) author as not hurt but enraptured, and the one time she is really hurt, she leaves him.

Symons’s remark in his book about what women have and men want implied that heterosexual sex is a scarce resource for which men strive, compete, and pay; he repeated it to a TV interviewer in a singles’ bar, where men are more likely to pay for the drinks and where there is no such thing as a “men’s night”—letting men in for free to attract women as paying customers. Roy Baumeister and Kathleen Vohs summarized the broader evidence about what is clearly a market in another paper, “Sexual Economics.” Most prostitutes are women who overwhelmingly serve male customers, and almost all male prostitutes serve men as well. Women rarely pay for sex, and when they do it often involves the pretense of a romantic tryst; they demand something more than a simple hookup—which is what men are typically paying for. When the famous actor Charlie Sheen was found to have paid top dollar for sex with one of the socalled Hollywood Madam’s young women, many wondered, Why would a man who could have his pick of willing women for free pay $1,500 (about $2,500 in today’s dollars) to have one come to his room? Answer: He doesn’t pay her to come to his room. He pays her to leave.

Strip clubs overwhelmingly consist of women performing for mainly male customers, although some women attend, often with male dates. There are about four thousand of these clubs in the United States, employing some 400,000 women. The reverse situation, in which men strip for women, represents a very small fraction of these numbers. There are also strip clubs for gay men, but very few for lesbians.

Finally, coercive sex is overwhelmingly male; 99 percent of FBI arrests for rape are of men. It is not obvious that it has to be this way because males have to be aroused to have sex; women could force men or other women to give them oral sex under threat of violence, as men do with both male and female victims, or use dildos or other objects to rape men anally, to humiliate them, as men do to victims of both sexes. Such assaults by women are vanishingly rare. A small percentage of gang rapes of women involve both sexes, and a small percentage is perpetrated by groups of women. Rape occurs in some lesbian relationships, but with nothing resembling its frequency in heterosexual and gay male relationships.

These are sound generalizations, not absolute rules. Some women do want sex as much as any man. Some men want little or none. There is nothing inferior about wanting it and nothing superior about not wanting it—although (despite the substantial minority with hypoactive sexual desire) women certainly have the potential for superior orgasmic capacity. But denial of the facts of human sexual nature as it applies to most men and women can only lead to confusion and, ultimately, to suffering. Male sexuality is driven. Men frequently want sex, period, while women tend to prefer it in the context of a relationship, a physical connection allied to an emotional one. Regardless of what I may privately desire, and regardless of how natural men’s needs may be, I can’t see that those divergent preferences are equally admirable.

To think that all these differences could result merely from cultural arrangements is naïve in the extreme. We now have overwhelming evidence that Elizabeth Cady Stanton’s “difference in man and woman”—by which she meant behavioral, psychological, and moral dispositions—is in part grounded in biology. Scientists once reticent in their assertions have become very bold. In 2011 the journal Frontiers in Endocrinology brought leading experts together. Psychologist Melissa Hines, for decades a respected researcher on the effects of prenatal hormones on gender, reaffirms their power but allows for two other newly proven influences: direct genetic effects on the brain, and the intrauterine environment. Neurobiologist Margaret McCarthy agrees that genes as well as hormones matter. She adds new evidence for how male and female become different in the hippocampus and amygdala—parts of the emotional brain outside the hypothalamus. Gender psychologists Sheri Berenbaum and Adriene Beltz show how exposure to high levels of prenatal androgens masculinizes later activity and occupational interests, sexual orientation, and some aspects of spatial ability, and they also find that pubertal hormones appear to influence gender identity and perhaps some male-female differences in psychiatric illness.

Neurobiologist Ai-Min Bao and her colleague Dick Swaab reviewed growing evidence for sex differences in the human hypothalamus and found that male-to-female transsexuals resemble women in these measures. They believe that male-female differences in cognition, gender identity, sexual orientation, and neuropsychiatric disorders are “programmed into our brain” very early on and, remarkably, that “there is no evidence that one’s postnatal social environment plays a crucial role in gender identity or sexual orientation.” Simon LeVay, who wrote the introduction for the special issue, had an established reputation as a neuroscientist studying the visual system when he came out to the world after showing that in one hypothalamic area gay men resemble women and differ from heterosexual men. Two decades later, he reconsiders the idea that social experience influences sexual orientation and gender identity, concluding that “little direct evidence supports this notion at present.”

The work is ongoing. Julia Sacher and her colleagues, in a 2013 summary of brain-imaging studies, found differences beyond the hypothalamus. Controlling for brain size, women have more gray matter and a thicker cerebral cortex. Women show stronger connectivity in the left brain, men in the right, contradicting expectations about men’s logic and women’s emotionality. Some studies find differences in the corpus callosum, a huge highway across the brain, which could mean the hemispheres collaborate better in women. Many studies show brain differences in activation of particular circuits in emotional and mental states, although this need not be causal. In 2014 Amber Ruigrok and her colleagues reanalyzed 126 brain-imaging studies and found several anatomical differences, although even such structural differences could theoretically result from upbringing. However, Sacher also showed in 2013 that there are many functional changes in brain activity over the menstrual cycle.

It’s important to understand that the similarities between men and women’s brains are much greater than any differences; the differences that exist are unrelated to general intelligence, but they are tied to specific dispositions. A key finding is that the male amygdala is relatively larger and dotted with testosterone receptors, while the prefrontal cortex, which inhibits aggressive and other impulses coming from the amygdala, is larger and develops earlier in women. These differences, combined with hormonal effects on the prenatal hypothalamus, could help explain why men greatly exceed women in violence and driven sexuality.

Incidentally, it used to be said that women could not be airline pilots or heads of state because of the supposed emotional swings of the menstrual cycle. That was before a landmark study, “Body Time and Social Time,” by sociologist Alice Rossi and economist Peter Rossi, the first and perhaps the only major menstrual cycle study that included men. For one thing, weekends had a much bigger impact than cycle phases. More importantly, men had the same number of bad days a month as women, except that the women’s were cyclical. So, would you rather have your airliner or your country piloted by someone who has bad days at random or someone who has the same number of bad days coming around like clockwork?

As for women being more emotional in general, we have seen that it depends on which emotions we are talking about. Women cry more easily, but male politicians tear up quite frequently in public. Women show more empathy in most situations, but men are far more likely to have violence on a hair trigger in international relations, and to feel and succumb to inappropriate and even destructive impulses while in office. Men are far more often distracted by sexual impulses and fantasies, even if these don’t result in problematic behavior. And if egotism and exaggerated ambition are emotional, which sex has more of those? All in all, the world will become safer and more efficient as women take their proper roles in leadership.

Excerpted from "Women After All: Sex, Evolution and the End of Male Supremacy" by Melvin Konnner, M.D. Published by W.W. Norton & Company. Copyright 2015 by Melvin Konner, M.D. Reprinted with permission of the publisher. All rights reserved.