If the world were ours too, if we believed we could get away with it,. . .the force of female desire would be so great that society would truly have to reckon with what women want, in bed and in the world.
—Naomi Wolf, The Beauty Myth, 1990
You’re at university and a stranger of the opposite sex sidles up to you. “I’ve been noticing you around campus. I find you to be very attractive,” they say. Before you know it, the mysterious person is inviting you back to their room to sleep with them.
It may be the least creative way of picking someone up, but if it works on you, then research suggests you’re almost certainly a man. This scenario was part of a real experiment at Florida State University conducted in 1978 and designed by psychology professors Russell Clark and Elaine Hatfield to settle a classroom dispute over whether, compared to women, men are more open to casual sex. Their method was simple. They recruited a bunch of young volunteers from an experimental psychology class, none of them too bad looking but none wildly attractive either, to approach people across campus and repeat the same pickup line. This was followed by one of three requests: to go out on a date, to go to their apartment, or to go to bed with them.
The results were stark. Even though men and women were equally likely to go on a date with a stranger, none of the women would sleep with one. Three-quarters of the men, on the other hand, were willing to have sex with a woman they didn’t know. When the psychologists repeated the experiment in 1982, the results were almost the same. The women, they observed, were often appalled at being propositioned in this way. “What is wrong with you? Leave me alone,” one said. The men were a different story, even apologizing when they refused. “In fact they were less willing to accept an invitation to date than to have sexual relations!” Clark and Hatfield noted.
For years they struggled to get their paper published for fear on the part of publishers that it was too frivolous. When it finally came out in 1989 in the Journal of Psychology and Human Sexuality under the title “Gender Differences in Receptivity to Sexual Offers,” it became a classic. After all, it neatly confirmed what everyone thought they already knew about sex and the sexes. Men are naturally polygamous and just fighting nature when they become tied into long-term relationships. Women are monogamous and always looking for the perfect partner.
It comes down to the fact, some biologists say, that males and females want fundamentally different things. They’re stuck in an endless evolutionary tussle—one indiscriminately chasing any female to boost his odds of fathering the most children, and the other trying to escape unwanted male attention in the careful search for the best-quality father for her offspring. Charles Darwin himself had laid this observation in scientific stone back in 1871 in his famous work The Descent of Man, and Selection in Relation to Sex.
The idea was even experimentally tested in 1948 in another mating experiment. This one wasn’t on humans, though, but on a humble little fly that appears when fruit rots.
When it comes to reproduction, the easiest species to study are those that mate quickly and breed abundantly. Humans are not that species.
Angus John Bateman, a botanist and geneticist working at the John Innes Horticultural Institute in London in 1948, was wise enough to pick the common fruit fly, a creature that lives so hard and fast that it’s sexually mature within a few days of birth and can lay hundreds of eggs at a time. What makes the fruit fly a scientist’s best friend is that it has genetic mutations that make each one look slightly different from the next, depending on what it inherits, such as curlier wings or narrower eyes. By tracking these differences, Bateman could reliably pick out which fly belonged to which parents. From this, he knew which flies were mating successfully.
Like Hatfield and Clark’s experiment, Bateman’s was simple. He took three to five adult females and the same number of adult males, then watched to see how they performed in the mating game. A fifth of the male flies, he found, didn’t manage to produce any offspring, compared with only 4 percent of the females. The most successful male flies, though, produced nearly three times as many offspring as the most successful female fly. None of the females were short of offers, but the least successful males suffered routine rejection. It confirmed Darwin’s long-standing theory that males in species like these are more promiscuous and less discriminating, while females are pickier and more chaste.
“Darwin took it as a matter of general observation that males were eager to pair with any female, whereas the female, though passive, exerted choice,” wrote Bateman. The fruit fly species he studied “seems to be no exception to the rule.”
Darwin had reasoned that when one sex has to compete for mates, there’s greater pressure on it to evolve the features the other sex is looking for. It needs to be strong enough to beat off the competition, too. He called this evolutionary process “sexual selection.” And his observations suggested that males faced far more of this pressure than females. This would explain why the males of certain species, including our own, tend to be bigger and stronger than the females. It explains, too, such marvels of nature as the lion’s giant mane and the peacock’s flamboyant blue and green plumage. There don’t seem to be any reasons why lions need manes or peacocks need such cumbersome, fancy feathers except to attract the opposite sex.
“There is nearly always a combination of an undiscriminating eagerness in the males and a discriminating passivity in the females,” wrote Bateman. His fruit fly experiment reinforced Darwin’s theory that sexual selection acts more heavily on males than on females. Some male flies were studs, others were duds, but none of them for want of trying. The competition was intense enough that a few did far better than the rest. The female flies, meanwhile, seemed to be comfortable in the knowledge that they could choose the males they wanted. They seemed to be under little pressure at all. In fact, according to Bateman, a tiny number were even willing to forgo mating for the moment if, presumably, they didn’t see what they liked.
Bateman’s observations of fruit flies, extrapolated to other species including our own, would renew scientific interest in sexual selection theory. But not immediately. His paper lay beneath the radar for decades. He never wrote about sexual selection again. It wasn’t until twenty-four years later that his fruit fly experiment was finally popularized by a young researcher called Robert Trivers.
Trivers, age seventy-three, has had a colorful life for a biologist.
His website, which promotes his autobiography—appropriately titled Wild Life—says that he’s spent time behind bars, that he founded an armed group to protect gay men in Jamaica from violence, and that he once drove a getaway car for a founder of the Black Panthers, the black nationalist organization active in the sixties and seventies. He was also the biologist who once told a reporter that biologist Sarah Blaffer Hrdy should focus on being a mother rather than on her career.
Today Trivers lives on a rural estate he’s bought in Jamaica. When I interview him over the phone, he tells me that he and the workers there call it “Man Town,” because there are no women around. When I ask him where he works nowadays, he says that he’s in a dispute with his employer, Rutgers University in New Jersey, which means that soon he’ll be out of a job. Apparently, he had been forced to teach classes on subjects he didn’t know anything about.
However much of a roller coaster his life has been, Trivers is considered one of the most influential evolutionary biologists in the world, in particular for theories he developed early in his career. A paper he published in 1972 about Angus Bateman’s 1948 fruit fly experiment has been cited by researchers at least eleven thousand times. Titled “Parental Investment and Sexual Selection,” it has fundamentally shaped the way researchers today understand sexual selection.
Trivers was just a young researcher at Harvard University, studying mating pigeons outside his window, when one of his tutors suggested he look up Bateman’s work. And he remembers it with graphic clarity. He went to the museum to photocopy it, “with my testicles firmly pressed against the side of the Xerox machine,” he tells me, with a throaty laugh. As soon as he read it, “The scales fell from my eyes,” he says. It would mark a turning point in his career.
He realized that females must be choosier and less promiscuous than males because they have a lot more to lose as parents from making a bad choice. Take the example of humans: men produce lots of sperm and don’t necessarily need to invest in their children, while women have only a couple of eggs to fertilize at a time, followed by nine months of pregnancy and many years of breast-feeding and child raising. “The logic was obvious after a moment’s reflection. You know the female is spending a lot producing those two eggs, and the male is spending a day’s ejaculate, which is trivial,” he explains. “When I lecture to students I sometimes point out that, during the last hour, every testicle in the room has generated a hundred million sperm. That’s a lot of sperm with nowhere to go.”
In his 1972 paper about Angus Bateman’s observations of fruit flies, Trivers writes, “A female’s reproductive success did not increase much, if any, after the first copulation and not at all after the second.” A female, he suggests, gains nothing from adding extra notches to her belt. One male is enough to get her pregnant, and once pregnant, she can’t be any more pregnant. “Most females were uninterested in copulating more than once or twice.”
This theory implies that when parental investment changes, so might sexual behavior. In monogamous species in which fathers are much more heavily involved in child care, these rules could theoretically reverse. The more that males invest time and energy in their children, the choosier they might become about whom they mate with and the more competitive females might become for their attention. And, indeed, in certain monogamous species of bird, it’s the females that chase after the males.
In humans, of course, many men are reliable fathers who invest as much as mothers in raising children. But Bateman didn’t believe this would necessarily change how men behave. He wrote that even in monogamous species with fairly equal numbers of males and females, the old pattern of sexual behavior—undiscriminating eager males and discriminating passive females—”might be expected to persist as a relic.” In his own paper, twenty-four years after Bateman’s, Trivers suggests, “In species where there has been strong selection for male parental care, it is more likely that a mixed strategy will be the optimal male course—to help a single female raise young, while not passing up opportunities to mate with other females whom he will not aid.”
In other words, he’s saying that men are unlikely to have escaped the evolutionary urge to cheat.
“Sounding sexist is not a good reason to ban a theory.”
The August 1978 issue of Playboy magazine carried a sensational story. “Do Men Need to Cheat on Their Women? A New Science Says Yes,” boasted the cover. The photograph next to the provocative headline coincidentally featured a model in white suspenders and strappy heels for an item on sexy secretaries. Her pad and pen were carelessly tossed to the floor while she stood pressed against her boss.
The publication of Robert Trivers’s paper marked a watershed not only in the way scientists understood sexual behavior but also in how the everyday woman and man in the street understood it. Sexual selection theory, revamped for the twentieth century, rapidly became a tool to explain women’s and men’s relationship habits. Bateman’s theories, once almost forgotten, were transformed into a fully blown set of universal principles, cited hundreds of times and considered solid as a rock. On that rock now rests an entire field of work on sex differences.
In 1979 prominent anthropologist Don Symons, now an emeritus professor at the University of California, Santa Barbara, in his seminal book The Evolution of Human Sexuality, reinforced the idea that men seek out sexual novelty while women look for stable, monogamous relationships. “The enormous sex differences in minimum parental investment and in reproductive opportunities and constraints explain why Homo sapiens, a species with only moderate sex differences in structure, exhibits profound sex differences in psyche,” he writes. One of Symons’s theories is that the female orgasm isn’t an evolutionary adaptation but a by-product of the male orgasm, just like male nipples are a vestige of female nipples. If women do experience orgasm, he implies that it’s only a happy biological accident.
An unimpressed critic at the time, Clifford Geertz at the Institute of Advanced Study, Princeton, summed up Symons’s book in the New York Review of Books with the old verse, “Higgamous, Hoggamous, woman’s monogamous; Hoggamous, Higgamous, man is polygamous.”
Despite the skepticism, within a couple of decades of his book being published, the science had gone mainstream. Robert Trivers’s work, by drawing human behavior further into the realm of evolutionary biology, had helped spawn an entire field of research known today as evolutionary psychology. One of the world’s most well-known academics in this subject is David Buss, who now teaches at the University of Texas at Austin. In his book The Evolution of Desire: Strategies of Human Mating, published in 1994, he writes, “Because men’s and women’s desires differ, the qualities they must display differ,” adding that it makes sense for women to be naturally monogamous because “women over evolutionary history could often garner far more resources for their children through a single spouse than through several temporary sex partners.”
This idea popped up again in a 1998 New Yorker article by the cognitive psychologist Steven Pinker. Under the title “Boys Will Be Boys,” he used evolutionary psychology to defend US president Bill Clinton, whose affair with his intern Monica Lewinsky had just been made public. “Most human drives have ancient Darwinian rationales,” he writes. “A prehistoric man who slept with fifty women could have sired fifty children, and would have been more likely to have descendants who shared his tastes. A woman who slept with fifty men would have no more descendants than a woman who slept with one.” Pinker has described Don Symons’s book as “groundbreaking” and Robert Trivers’s work as “monumental.” He was also among those who stood up for Harvard University president Lawrence Summers when he suggested that innate sex differences might explain the shortfall of top female scientists.
The scope of Charles Darwin’s original work on sexual selection stretched far beyond sexual behavior, of course. It wasn’t just about mating habits but also about how the pressure to attract the opposite sex would have acted more heavily on males, influencing their evolutionary development by forcing them to become more attractive and smart. In the Descent of Man in 1871, he wrote, “The chief distinction in the intellectual powers of the two sexes is shewn by man attaining to a higher eminence, in whatever he takes up, than woman can attain. . . . Thus man has ultimately become superior to woman.”
More than a century later, even this controversial aspect of sexual selection theory has been resurrected. In 2000, an evolutionary psychologist at the University of New Mexico, Geoffrey Miller, published The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature, in search of what he calls “a theory for human mental evolution.” Females in our evolutionary past may have developed a preference for males who were better at singing or talking, he writes. As men became more creative and intelligent and better at singing and talking, they would have become more attractive and successful at mating. Through a “runaway process” in which smarter males mated more often and sired smarter offspring, Miller argues, the human brain could have reached its relatively large size as quickly as it did.
“Male nightingales sing more and male peacocks display more impressive visual ornaments. Male humans sing and talk more in public gatherings, and produce more paintings and architecture,” he writes. Later he adds, “Men write more books. Men give more lectures. Men ask more questions after lectures. Men dominate mixed-sex committee discussions.” Men are better at all these things, he implies, because they have evolved to be better.
For anyone who fears this might be a little unfair to women, Miller has a response. “In the game of science,” he advises his readers, “sounding sexist is not a good reason to ban a theory.”
“Multiple mating is very, very common among females.”
At the heart of sexual selection theory, as it applies to humans at least, is the notion that men are promiscuous and undiscriminating while women are highly discriminating and sexually passive. Females are choosy and chaste. It all comes down to Angus Bateman’s principles, as demonstrated both by his flies and by Clark and Hatfield on the campus of Florida State University in 1978. Men will sleep with strangers while women simply won’t.
But not everyone is convinced this is true.
Today there is a huge body of research that flies in the face of Bateman’s principles. It has been building up for many decades. Anthropologist and primatologist Sarah Hrdy’s research on the Hanuman langurs of Mount Abu forty years ago showed that a female monkey can benefit from mating with more than one male because it confuses them all over their possible paternity of her children, making them less likely to commit infanticide. In her vivid studies of red colobus monkeys in the Abuko Nature Reserve in Gambia, London-based anthropologist Dawn Starin also describes how sexually confident female primates can be. “When it came to sex, she was nothing if not assertive,” she writes in a 2008 issue of Africa Geographic, about a monkey she saw. “For a few months every year, the forest is taken over by a bunch of female hooligans, strutting their stuff, giving guys the eye and luring nervous males into the bushes.”
In more distant species from us, researchers have found similar evidence of females mating with multiple males. Many birds thought to be monogamous have turned out not to be. Female bluebirds have been spotted flying considerable distances at night just to mate with other males. Data on the small-mouthed salamander, bush crickets, yellow-pine chipmunks, prairie dogs, and mealworm beetles have shown that the females of all these species, too, enjoy more reproductive success when they mate with more males.
“It’s pretty widespread. Some would even say ubiquitous. Multiple mating is very, very common among females,” says animal behaviorist Zuleyma Tang-Martínez from the University of Missouri, Saint Louis. She tells me that as a graduate student she was as convinced of Bateman’s logic as anyone. “It’s a very simple idea. It makes sense in terms of the cultural stereotypes we have, and so you buy into it,” she says. “It was only when I sort of matured as a scientist that I started asking questions, and I started seeing evidence come out that didn’t go along with Bateman, that I started to take a much more thorough look at the evidence.”
Tang-Martínez has spent years dissecting the facts around Bateman’s principles and published numerous papers on his ideas. Her conclusion is that the sheer weight of evidence should be enough to force scientists to rethink Bateman’s principles. In fact, she adds, a paradigm shift is already underway. Scientific understanding around the breadth of female sexual nature has expanded to better encompass the true variety in the animal kingdom. Far from being passive, coy, and monogamous, females of many species have been shown to be active, powerful, and very willing to mate with more than one male.
However, the shift has been slow to come in part because of huge amounts of resistance along the way. In his 1982 review of Sarah Hrdy’s book The Woman That Never Evolved—which presents more evidence contradicting the image of the coy, chaste female—anthropologist Don Symons raised his eyebrows, especially at her suggestion that, like the female langurs at Mount Abu, evolution might favor females that are sexually assertive and competitive. “In promoting her view of women’s sexual nature, Hrdy provides dubious evidence that this nature exists,” Symons wrote, dismissively.
According to Sarah Hrdy, this hostility toward viewpoints like hers hasn’t gone away. “It is impossible to understand this history without taking into account the background, including the gender, of the researchers involved,” she wrote in a chapter of Feminist Approaches to Science, published in 1986. In her own review of Don Symons’s book on human sexuality from 1979, she referred to this old-fashioned way of thinking as “a gentlemanly breeze from the nineteenth century.” She believes that, just like in Darwin’s time, scientists have twisted sexual selection theory in ways that are unfair not only to women but also to the truth.
“Sexual selection is brilliantly insightful. Darwin got that exactly right. The problem was that it was too narrow and it didn’t explain everything,” Hrdy tells me. Some of the most powerful evidence against Bateman’s principles isn’t even in other species but in our own, adds Zuleyma Tang-Martínez. “If there’s any place that I think I would be extremely reluctant, to put it mildly, to say that Bateman applies, it would be humans,” she warns. “I think it’s a huge mistake.”
“Around half of societies say female infidelity is either common or very common,” says Brooke Scelza, a human behavioral ecologist at the University of California, Los Angeles. She has a playpen in the corner of her office, and as a young working mother myself I immediately empathize with her.
It’s Scelza’s empathy with women, in turn, that has given her a unique insight into the cultures she has studied around the world. They include the Himba, an indigenous society of partly nomadic livestock farmers living in northern Namibia. The reason the Himba are vital to understanding the true breadth of female sexuality is because on the spectrum of sexual freedom, Himba women are at a far end. Their culture has a relaxed attitude to women having affairs with other men while they’re married, offering them more autonomy and choice over who they have sex with than women in almost any other part of the world.
Carrying out interviews about their marital history, Scelza found that Himba women would tell her which children were fathered by their husbands, but then use the local word omoka to describe their other children. “It means you get your water from someplace else. So it’s a euphemism. Basically, it’s a word they use to describe a child that’s either born out of wedlock or who is born through an affair,” explains Scelza. Husbands, too, would admit quite openly which of their wives’ children they thought were their own and which they thought were someone else’s.
Although there’s no reason to think men and women don’t feel jealous, adds Scelza, the cultural norm among the Himba is that it’s as acceptable for women to have affairs as it is for men, and husbands simply have to accept them. They profoundly challenge Angus Bateman’s theory that women aren’t eager for sex or that they don’t want more than one sexual partner at a time.
When Scelza started doing fieldwork with the Himba in 2010, women would ask her why she didn’t have men coming to her hut. “Well, I said, ‘You know, I’m married.’ And they said, ‘Yeah, yeah, but that doesn’t matter. He’s not here.’ So then I tried to explain that my marriage was a love match, because then I thought they would understand. And they said, ‘It doesn’t matter. It’s okay, it’s okay. He’s not going to know; it’s okay,’” she recalls. “They really hold a very different idea in their heads about love and sex, that it wouldn’t be a bad thing at all for me to say, on the one hand, that I really love my husband but that I’ll still be having sex with somebody else when we’re apart. That, to them, was not a transgression.”
In his 1972 paper on Bateman’s fruit fly experiment, biologist Robert Trivers had said that this behavior could have no evolutionary benefits for females. One man is enough to get a woman pregnant, and this marks the limit of her reproductive capacity. More lovers can’t make her any more successful at having children. But Scelza has found that statistically this isn’t true. “It turned out that having some kids through affairs was actually good for your overall reproduction,” she explains.
She’s still in the process of collecting data and figuring out the reasons for this. It may be no more than a random correlation, perhaps because the most fertile and highest quality women, who would have more children anyway, attract the most partners. Another factor, of course, is that not every man is as fertile or as good a father as the next. But she adds that there are other reasons why births and child survival go up as women mate with more men. Economics, for example; they might bring in more resources or protection.
Another one is sexual compatibility. Among the Himba, arranged marriages are common, which means women don’t always get the husband of their choice. Affairs offer them a workaround by giving them the benefit of a committed, reliable husband at home, as well as the man or men they are more sexually compatible with, away from home.
There’s some early research indicating, in other species at least, that when a female chooses the male she wants, her offspring are more likely to survive. In 1999, at the annual meeting of the Animal Behavior Society, Patricia Gowaty, who was then at the University of Georgia, and Cynthia Bluhm at the Delta Waterfowl and Wetlands Research Station in Manitoba, Canada, reported this effect in female mallard ducks. Mallards form pair bonds, but male ducks often viciously harass females into mating with them. When a female duck was allowed to choose her mate free from harassment, her ducklings survived better, Gowaty and Bluhm told Science News. Gowaty, working with another team, has seen similar results in house mice.
The Himba, however, are just one band in the rainbow of human behavior. Himba women have the sexual freedom they do partly because of the unusual way in which their society is organized. Women keep close ties to their mothers and childhood families after they get married, which makes it easier for them to leave their husbands and do what they want without disapproval or control. Also, wealth isn’t passed down from a father to his children but from a brother to his brother or to his sister’s sons, which means that a man may be less concerned with knowing his children are his own. Whoever inherits his cows is guaranteed to be a genetic relative.
In a 2013 paper published in the journal Evolutionary Anthropology, “Choosy But Not Chaste: Multiple Mating in Human Females,” Brooke Scelza lists a few other places in which women have more than one partner. The Mosuo of China, one of the few societies in the world in which women head households and property is passed down the female line, people practice what is known as “walking marriage.” This allows a woman to have as many sexual partners as she likes. The lover of her choice simply comes to her room at night and leaves the next morning. What marks the Mosuo apart is that men traditionally don’t provide much economic or social support to their children.
Similarly, in other small-scale societies where women contribute more to the family plate, women tend to have more sexual freedom. In the United States, notes Scelza, “in sub-populations in which reliability on male resources is low as a result of high incarceration rates and unemployment, female kin provide critical instrumental and emotional support, and patterns of serial monogamy are common.”
Another example is in South America, where some isolated societies practice partible paternity, the belief that more than one man can be the father of a baby. In a paper on the topic in the journal Proceedings of the National Academy of Sciences in 2010, Robert Walker and Mark Flinn at the University of Missouri, Columbia, and Kim Hill at Arizona State University write, “On the universal partible paternity end of the spectrum, nearly all offspring have purported multiple cofathers, extramarital relations are normal, and sexual joking is commonplace.”
Tracking reproductive success in populations across the world, including Finland, Iran, Brazil, and Mali, researchers Gillian Brown and Kevin Laland at the University of St. Andrews and Monique Borgerhoff Mulder at the University of California, Davis, similarly found huge variation. In their paper published in the journal Trends in Ecology and Evolution in 2009 they said the data are “inconsistent with the universal sex roles that Bateman envisaged.”
All this, says Scelza, punctures the biological model of the coy, chaste female. Working with the Himba, who have a sexual culture so different from her own, has taught her that the rules about how women and men behave in relationships have far more to do with society than biology. The Himba aren’t a breed apart. They’re just culturally different. “It’s not that they don’t have love. It’s not that sex has replaced love in this society. They feel jealous. But the cultural norms that are in place prevent men from really being able to act upon it,” she explains. “If he was, for example, to hit his wife or something like that, which in some places in the world is completely an acceptable response, there would be a backlash. He would probably end up having to pay a fine and be punished for that action.”
If there is a difference in sexual behavior, adds Scelza, it’s that Himba women seem to be more discriminating than the men. “I think they’re still being picky. But I think being picky doesn’t mean one partner and you have to stick with them for life.”
Where does this all leave Angus Bateman’s cherished principles?
As more evidence rolls in, researchers have started to further question the scientific orthodoxy that females are generally more passive and chaste than males. Even the famous 1978 experiment on the campus of Florida State University—which found that men were overwhelmingly more open than women to casual sex with strangers—has been repeated, with surprising results.
“I felt like it wasn’t telling the whole story,” explains psychologist Andreas Baranowski from Johannes Gutenberg University. In the summer of 2013, he and colleague Heiko Hecht decided to run Clark and Hatfield’s seminal study again, this time controlling for certain factors they felt might have affected the original outcome. They were driven by their own personal observations of dating and sex. They instinctively didn’t believe that the Florida State University experiment had captured the true spectrum of how women behave. “It wasn’t what my experience was in Germany here, or in Europe in general. And also of other colleagues and friends,” Baranowski tells me. “My female friends would tell me about hookups and stories about how they would engage in sexual relationships with men, and that’s also not represented in the data at all. So it was a bit like, that is weird.”
Baranowski and Hecht suspected that women might reasonably be put off having sex with a stranger for lots of good reasons, including the social stigma of getting picked up so casually and, more obvious, the risk that they might be attacked. “We wanted to find out how the original findings would stand up to a more naturalistic setting, such as a cocktail bar, and a more safe setting, namely a laboratory,” they wrote in their paper, published in the journal Archives of Sexual Behavior in 2015. They wanted to make sure they didn’t veer too far from the original experiment either, so they ran it on a university campus as well.
Both on the campus and in the cocktail bar, they got fairly similar results to Clark and Hatfield, with slightly more men than women agreeing to a date, and many more men agreeing to sex. In both cases, though, men weren’t nearly as keen to go on dates or have sex compared to the Florida State University experiment. It wasn’t proof that Clark and Hatfield had got it wrong, but it was certainly evidence that different places and times can yield different results.
And this was crucial in showing that there’s no one way in which the sexes typically behave. The original experiment just wasn’t representative. “It’s really one dimensional, representing the dating market in the United States on a university campus in the seventies. That’s how I felt about it,” says Baranowski. “I didn’t doubt that they did proper protocol. I think they did. It’s just a microcosm there, where they did the experiment.”
Where their data got really interesting, though, was in the lab. They wanted their subjects to believe they were going on genuine dates with real people, so the researchers concocted an elaborate ruse based on a dating study. Each person was shown ten photographs of strangers of the opposite sex and told that all these strangers wanted to go on a date or meet up for sex with the subject in particular. If they agreed to meet, they were given a safe environment, and Baranowski and Hecht’s research team would then film the first half of their encounter.
All the men in the study agreed to go on a date and also have sex with at least one of the women in the photographs. For women, the figure was 97 percent agreeing to a date and, unlike the first experiment, “almost all women agreed to have sex,” says Baranowski.
It was evidence, they noted in their paper, that gender differences are significantly smaller in a nonthreatening environment. It may not have been biology holding women back in the Florida State University experiment but other reasons, most likely social and cultural—like the fear of violence or a moral double standard. One sex difference they did notice in the laboratory setting, though, was that women tended to pick out fewer partners from the photographs they were offered. Like Brooke Scelza found with the Himba in Namibia, they were choosier than the men, but not less chaste.
“We can’t just go on pretending that everything is hunky-dory.”
“Things like Bateman’s principles actually don’t make sense to me,” says Patricia Gowaty, distinguished professor at the University of California, Los Angeles.
We’re sitting on the patio of her home on a mountain in Topanga, nestled in a sprawling state park in Los Angeles County. We’re surrounded by wildlife. At one point during our meeting, a wild deer wanders nearby. Gowaty is an animal expert, an evolutionary biologist and a firebrand who has spent her career, which spans five decades, to leaching sexism out of her field by challenging its basic assumptions. Her most famous target has been Angus Bateman’s 1948 experiment showing that male fruit flies are more promiscuous than females.
“I became a scientist at the same time as I was becoming a feminist. They were coincident,” she tells me. Gowaty’s feminism has never waned. It influences her now as much as it did when she had her first job in the education department at the Bronx Zoo in New York in 1967. “In the late 1960s, all over the country, there were groups that were coming together to consciousness raise. The idea of consciousness-raising was simply to talk and to bring to consciousness the ideas associated with the feminism that was emerging at that time.” Through forums like these, she began to understand how women throughout history, including her own mother, had been constrained. Their achievements were against the odds.
“There are many women of my generation who have published with their initials to hide their gender,” she tells me.
Gowaty was angered, just as her contemporaries Sarah Hrdy and Adrienne Zihlman were, by how evolutionary biology was ignoring and misunderstanding women. Bateman’s principles lay beneath some of the claims that angered her most. She spent thirty years studying the mating behavior of Eastern bluebirds, and in the 1970s, when she suggested that female birds were flying away to mate with males that weren’t their partners, she simply wasn’t believed. Her male colleagues couldn’t accept it. They told her instead that the female bluebirds must have been raped.
“I think one of the things that Bateman’s principles do is they obfuscate variation in females. So suddenly, there’s nothing interesting about females. That’s one of the things that bothers me about it. There’s embedded sexism there, I think,” she says. “They may as well be tenets of the faith.”
Gowaty knew that the ultimate test of any scientific experiment rests on the ability to replicate it. So in the 1990s, after studying Bateman’s paper in detail, she decided it was time to do exactly that. What she and her colleagues at the University of Georgia, Rebecca Steinichen and Wyatt Anderson, found contradicted Bateman in the most fundamental way. “We observed the movements of females and males in vials during the first five minutes of exposure to one another. Video records revealed females went toward males as frequently as males toward females; we inferred that females were as interested in males as males in females,” they wrote in their paper, published in the journal Evolution in 2002.
This raised the dilemma of just how Bateman managed to see what he claimed to see in his own fruit flies. Investigating further, Gowaty soon began to notice problems with Bateman’s study. In a subsequent paper, published in 2012 in Proceedings of the National Academy of Sciences, Gowaty and researchers Yong-Kyu Kim and Wyatt Anderson at the University of Georgia, wrote, “Bateman’s method overestimated subjects with zero mates, underestimated subjects with one or more mates, and produced systematically biased estimates of offspring number by sex.” They claim that Bateman counted mothers as parents less often than fathers, which is a biological impossibility, since it takes two to make a baby.
Another error is that the same genetic mutations Bateman needed his flies to have so he could distinguish the parents from their offspring also affected the fruit flies’ survival rates. A fly with two severe and debilitating mutations, such as uncomfortably small eyes and deformed wings, could have died before Bateman had the chance to count it. This would have almost certainly skewed his results, too.
The mistakes are so clear, claims Gowaty, that Bateman’s 1948 paper could only have been published if the editor—who should have checked for errors—hadn’t actually read it. Failure to replicate scientific findings is a big deal. Often it leaves grave doubts about the original experiment. And for an experiment as important as Bateman’s it should cause enormous concern.
In this case, though, the reaction to her findings has been mixed. “A lot of people were very excited about it, other people were pissed about it. . . . It was like they were mad,” she tells me. When I e-mail Don Symons, who wrote The Evolution of Human Sexuality in 1979, to ask his opinion on Gowaty’s failure to replicate Bateman’s findings, he tells me he hasn’t read her paper. When I ask instead for his broader thoughts on the evidence of multiple mating in females, he tells me that he’s no longer available to answer my questions for personal reasons.
I also ask Robert Trivers, who first popularized Bateman’s paper in 1972, for his response. “I was afraid you were going to ask that,” he tells me over the phone from Jamaica. “I have not read the God Jesus paper.” He agrees to look at it for me, but doesn’t get around to reading it thoroughly even after a few weeks. “Since Patty is a careful scientist my bias is that she is correct,” he finally tells me by e-mail. Even so, he adds that research on other species (including his research on a Jamaican giant lizard) has reinforced Bateman’s principles. He sends me a paper published a couple of months earlier in the journal Science Advances by a team of European and US researchers. It reviews examples from more than a century of animal data, concluding, “Sexual selection research over the last 150 years has not been carried out under false premises but instead is valid and provides a powerful explanation for differences between males and females.”
For Gowaty, this defense isn’t enough. Picking out examples in the animal kingdom that happen to be consistent with Bateman’s principles ignores the wealth of inconsistencies—including, it seems, fruit flies. If there is enough contradictory evidence, this should put the underlying theory in doubt. The principles can’t be considered principles if there are so many exceptions. The problem is that Bateman’s and Trivers’s ideas have taken on such a life of their own that this no longer appears to make much difference. “I think people are hung on Bateman’s principles. They say that the principles stand whether the data are right or not,” says Gowaty.
The failure of prominent scientists such as Symons and Trivers to read her work when it was published makes it even more difficult for Gowaty to make the wider scientific community aware of her findings.
“I find it tremendously strange,” says animal behaviorist Zuleyma Tang-Martínez. “When a paper like that comes out, you would think that people who are interested in the topic would read it, regardless of which side they’re interested in or which side they tend to agree with. I try to read papers by people who don’t agree with my position. And I can’t imagine just saying, ‘Oh, I didn’t bother to read it.’ That to me seems almost insulting to a fellow scientist, to take that attitude.”
For Gowaty, this is more than a professional frustration. “I think that our inability to see alternatives is associated with our commitment to see sex differences. The canon of sex difference research is about sex roles and the origin of sex roles and the fitness differences that supposedly fuel those. These arguments are the ones that we really need to understand in order to make inferences that are reliable. I happen to think that the canon is flawed, and it’s flawed because it starts with sex differences to predict other sex differences. It is essentialist,” she explains.
“Many of these theories that we have in evolutionary biology about sex differences are not fundamental theories. They’re hand-wavy as hell.”
That’s not to say Bateman was completely wrong. Only that he wasn’t entirely right. If we were to judge Angus John Bateman’s principles today, it’s likely that the jury would be out. “I think, certainly, there are species that fit that mold,” says Tang-Martínez. In a review of evidence she published in the Journal of Sex Research in 2016, she lists the red-backed spider, pipefish, and seed beetles as examples of creatures that support Bateman’s hypothesis.
“But I do think that given the amount of evidence all the way across the board, from male investment and cost of sperm and semen, all of the sort of original underpinnings of his whole idea, that we have to rethink,” Tang-Martínez adds. “We can’t just go on pretending that everything is hunky-dory, and that we can still apply Bateman across the board to all species.”
She describes his principles as a box. As time wears on, fewer species—including humans—seem to fit in the box. Indeed, it’s possible to argue that if ever there was proof that females aren’t naturally chaste or coy, it’s the extraordinary lengths to which some males go to keep them faithful.
“Let me tell you one anecdote from birds,” Robert Trivers tells me.
It’s from his graduate student days, when he would watch the pigeons on the gutter outside his third-floor window. In the winter, the birds would huddle together in rows for warmth. “You have two couples sitting next to each other in winter. They may have sex in December, but it’s nonreproductive, trust me. Throughout the winter they’re not having sex together; they’re just staying together, and they intend to breed together in the spring as soon as breeding season arrives,” he begins.
The issue for the males is how to make sure they don’t lose their female partners to another male. Trivers imagines himself as one of the male pigeons. “If you have four individuals sitting next to each other, then the males sit on the inside, even though they are the more aggressive sex,” he explains. “I sit in between the other male, who sits to my right, and my female sits to my left. He, meanwhile, has his female to his right. So both of us can relax during the night. We’re in between any other male and our female.” This arrangement means that each male can successfully protect his female from unwanted attention from the other male in their huddle.
But a dilemma sets in when another couple is added to the mix. With three males and three females, things get complicated. “Now it’s impossible to have a seating arrangement such that each male is between his female and all the other males,” he says. “So what you get instead is the outer two, the far left male and the far right male, each have their mate on the outside of them. So they’re protecting their mate from contact with the other males.” This leaves one male in a quandary. “Now, what about the central male? What does he do?” he asks me. “What he does is he pecks his female and forces her to sleep on the slanting roof several inches above him and several inches above the seat she would prefer to be on, which is sitting on the gutter, on which she would have a male on both sides of her.” The male forces her to sit alone uncomfortably in the cold.
As a student, Trivers would sometimes work until three in the morning. “So at one thirty, I would hear some ‘woo hoo-hoo,’ and I would see, ha ha! What happened is the male has fallen asleep and the female has crept back down to the comfortable position, which is how she would prefer to sleep the night. He wakes up and sees she is there, and pecks her back up into this uncomfortable position!” he says. “The sexual insecurity or the risk of an extra-pair copulation is strong enough to make me willing to inflict a cost on my mate.”
This phenomenon may seem bizarre—cruel, when seen through human eyes—but it’s common across many species, including our own. It’s known as “mate guarding.” It’s a vitally important piece of the puzzle when it comes to understanding relationships and the balance of power between females and males. Even though it might well harm the male to have his partner so distressed through the winter, leaving her with less energy come spring when she would need to reproduce and look after their offspring, he doesn’t stop pushing her away from the other males. It’s more important to him that he doesn’t lose her to another pigeon, even for a moment.
For Trivers, this is powerful evidence of intense male competition for females. But seen from a different point of view, it also casts the underlying assumptions of Charles Darwin and Angus Bateman in an alternative light. Male sexual jealousy, the fear of being cuckolded, and such vicious mate guarding suggest that females aren’t naturally chaste or passive at all. If they were, then why would their partners go to such extraordinary lengths to stop them getting anywhere near other males?