by Angela Saini
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.