THE NEW SCIENCE OF SEX AND GENDER
The notion that behavioral differences between the sexes are innate and immutable does not hold up under scrutiny
ONE OF AUSTRALIA'S MORE PROVOCATIVE ART MUSEUMS, the Museum of Old and New Art in Hobart, Tasmania, recently hosted an exhibition on the evolution of art. Three evolutionary scientists who guest-curated the show offered their perspectives on how evolution explains not just the characteristics of amoebas, ants and antelopes but also the uniquely human endeavor of art. One of these explanations sees art as an evolved trait akin to the peacock's effervescently colored tail, which increases its bearer's reproductive success by signaling superiority as a mate.
Hands up if this scenario conjures in your mind the image of a much feted female artist, famous for fearlessly pushing the boundaries of artistic convention, pleasurably making her way through a series of handsome young male muses? We didn't think so.
The stereotype of the daring, promiscuous male—and his counterpart, the cautious, chaste female—is deeply entrenched. Received wisdom holds that behavioral differences between men and women are hardwired, honed by natural selection over millennia to maximize their differing reproductive potentials. In this view, men, by virtue of their innate tendencies toward risk-taking and competitiveness, are destined to dominate at the highest level of every realm of human endeavor, whether it is art, politics or science.
But a closer look at the biology and behavior of humans and other creatures shows that many of the starting assumptions that have gone into this account of sex differences are wrong. For example, in many species, females benefit from being competitive or playing the field. And women and men often have similar preferences where their sex lives are concerned. It is also becoming increasingly clear that inherited environmental factors play a role in the development of adaptive behaviors; in humans, these factors include our gendered culture. All of which means that equality between the sexes might be more attainable than previously supposed.
FAST MALES, FINICKY FEMALES
THE ORIGIN of the evolutionary explanation of past and present gender inequality is Charles Darwin's theory of sexual selection. His observations as a naturalist led him to conclude that, with some exceptions, in the arena of courtship and mating, the challenge to be chosen usually falls most strongly on males. Hence, males, rather than females, have evolved characteristics such as a large size or big antlers to help beat off the competition for territory, social status and mates. Likewise, it is usually the male of the species that has evolved purely aesthetic traits that appeal to females, such as stunning plumage, an elaborate courtship song or an exquisite odor.
It was, however, British biologist Angus Bateman who, in the middle of the 20th century, developed a compelling explanation of why being male tends to lead to sexual competition. The goal of Bateman's research was to test an important assumption from Darwin's theory. Like natural selection, sexual selection results in some individuals being more successful than others. Therefore, if sexual selection acts more strongly on males than females, then males should have a greater range of reproductive success, from dismal failures to big winners. Females, in contrast, should be much more similar in their reproductive success. This is why being the animal equivalent of a brilliant artist, as opposed to a mediocre one, is far more beneficial for males than for females.
Bateman used fruit flies to test this idea. Although the technology for paternity testing did not exist at the time, he inferred parentage and the number of different mates of males and females as best he could. He did this rather ingeniously, by using fruit flies with different genetic mutations, including one that makes the bristles on the wings extra long, another that makes the wings curl upward, and yet another that renders the eyes very small or absent. These mutations are sometimes evident in offspring, so Bateman could estimate how many offspring each adult produced by counting the number of different mutants among the surviving offspring. From his data, he concluded that males were indeed more variable than females in their reproductive success (measured as offspring). Bateman also reported that only male reproductive success increased with the number of mates. This result, he argued, is why males compete and females choose: a male's reproductive success is largely limited by the number of females he can inseminate, whereas a female reaches her plateau with a single mate that provides her with all the sperm she needs.
Scholars mostly ignored Bateman's study at first. But some two decades later evolutionary biologist Robert Trivers, now at Rutgers University, catapulted it into scientific fame. He expressed Bateman's idea in terms of greater female investment in reproduction—the big, fat egg versus the small, skinny sperm—and pointed out that this initial asymmetry can go well beyond the gametes to encompass gestation, feeding (including via lactation, in the case of mammals) and protecting. Thus, just as a consumer takes far more care in the selection of a car than of a disposable, cheap trinket, Trivers suggests that the higher-investing sex—usually the female—will hold out for the best possible partner with whom to mate. And here is the kicker: the lower-investing sex—typically the male—will behave in ways that, ideally, distribute cheap, abundant seed as widely as possible.
The logic is so elegant and compelling it is hardly surprising that contemporary research has identified many species to which the so-called Bateman-Trivers principles seem to apply, including species in which, unusually, it is males that are the higher-investing sex. For example, in some species of katydids, also known as bush crickets, the male's investment in reproduction is greater than the female's, thanks to a nutrient-rich package he provides, along with sperm, during copulation. Females thus fight one another for access to males.
The Bateman-Trivers principles also seem to provide a plausible explanation of the gender dynamics of human societies. Women are commonly understood to have less interest in casual sex with multiple partners, for instance, and to be more caring and less competitive and risk-taking. Applying the Bateman-Trivers logic, these behaviors serve to protect their investment. Contemporary advice from Facebook's chief operating officer Sheryl Sandberg to women to "lean in" at work to rise to the top thus appears to be undercut by arguments that predispositions to take risks and compete have evolved more strongly in males than in females because of greater reproductive return.
BREAKING THE RULES
BUT IT TURNS OUT that nature is not nearly so simple and neat as this line of reasoning would suggest, even for nonhuman animals. In the decades since the Bateman-Trivers principles were forged, many of their foundational assumptions have been overturned. One such change in thinking concerns the supposed cheapness of reproduction for males. Sperm is not always cheap, nor is it always abundant: for instance, male stick insects can take several weeks to recover their libido after a lengthy copulation. And more recent scrutiny of the fruit fly's reproductive habits found that males do not always take up mating opportunities. Male selectivity has consequences for females of many insects, because if they mate with a male that has copulated extensively, they risk acquiring insufficient sperm. Scarce or limited sperm is not an uncommon challenge for females, which may mate repeatedly with different males precisely to acquire enough sperm.
In fact, a reexamination of Bateman's data from the lab of Patricia Gowaty of the University of California, Los Angeles, revealed, crucially, that a female fruit fly's reproductive success also increased with her mating frequency, a pattern that has emerged for a great many other species of animals. Furthermore, field studies show that mating for females is not the given scientists once assumed it to be. In a surprisingly large number of species, a significant proportion of females do not encounter a male and are thus unable to reproduce. Nor is promiscuous mating standard practice for males. Monogyny, in which males mate only once, is not uncommon and can be an effective means of maximizing reproductive success.
Insects are not the only creatures that challenge the Bateman-Trivers principles. Even in mammals, for which investment in reproduction is particularly skewed because of the costs of gestation and lactation for females, competition is important not just for male reproductive success but also for female reproductive success. For example, the infants of higher-ranking female chimpanzees have higher rates of both arrival and survival than those of lower-ranking females.
In our own species, the traditional story is additionally complicated by the inefficiency of human sexual activity. Unlike many other species, in which coitus is hormonally coordinated to a greater or lesser degree to ensure that sex results in conception, humans engage in a vast amount of nonreproductive sex. This pattern has important implications. First, it means that any one act of coitus has a low probability of giving rise to a baby, a fact that should temper overoptimistic assumptions about the likely reproductive return on seed spreading. Second, it suggests that sex serves purposes beyond reproduction—strengthening relationships, for example.
Cultural and societal changes further necessitate rethinking the application of Bateman-Trivers principles to humans. The dichotomous view of the sexes that held sway in the last century has given way to one that sees differences mainly in degree rather than kind. Increased female sexual autonomy wrought by the birth-control pill and the sexual revolution has led to marked increases in premarital sex and numbers of sexual partners in women especially. And women and men report largely similar preferences for their sex lives. For example, the second British National Survey of Sexual Attitudes and Lifestyles, based on a random sample of more than 12,000 people between the ages of 16 and 44 surveyed around the turn of this century, found that 80 percent of men and 89 percent of women preferred monogamy.
Meanwhile the feminist movement increased women's opportunities to enter, and excel in, traditionally masculine domains. In 1920 there were just 84 women studying at the top 12 law schools that admitted women, and those female lawyers found it nearly impossible to find employment. In the 21st century women and men are graduating from law school in roughly equal numbers, and women made up about 18 percent of equity partners in 2015.
RISKS AND BENEFITS
AS WE ZOOM IN from this broad-brush perspective on gender patterns to a fine-grained examination of sex differences in behavior, the familiar evolutionary story becomes even muddier. Consider risk-taking, once assumed to be a masculine personality trait, thanks to its role in enhancing male reproductive success. It turns out that people are quite idiosyncratic in the kinds of risks they are willing to take. The skydiver is no more likely to gamble money than the person who prefers to exercise in the safety of the gym. It is people's perception of the potential costs and benefits of a particular risky action, not their attitude toward risk per se, that explains their willingness to take risks. These perceived costs and benefits can include not only material losses and gains but also less tangible impacts on reputation or self-concept.
This nuance is important because sometimes the balance of risks and benefits is not the same for men and women because of physical differences between the sexes or gendered norms, or both. Consider, for example, the risk of a casual sexual encounter. For a man, the gains include the near certainty of an orgasm and perhaps a burnishing of his reputation as a "stud." For a woman, sexual pleasure is far less likely from casual sex, according to a large-scale study of North American students published in 2012 by Elizabeth Armstrong of the University of Michigan and her colleagues. And thanks to the sexual double standard, her reputation is more likely to be damaged by the episode. Among young Australians, for example, sociologist Michael Flood, now at the Queensland University of Technology, found that the label "slut" retains a stronger "moral and disciplinary weight … when applied to women." Moreover, a woman bears greater physical risks, including pregnancy, sexually transmitted disease and even sexual assault.
The lens of different risks and benefits can also clarify the sexes' different propensity to assert themselves at work, as Sandberg has advised women to do. It is hard to see how a young female lawyer, looking first at the many young women at her level and then at the very few female partners and judges, can be as optimistic about the likely payoff of leaning in and making sacrifices for her career as a young male lawyer. And this is before one considers the big-picture evidence of sexism, sexual harassment and sex discrimination in traditionally masculine professions such as law and medicine.
Still, the idea that a nonsexist society could erase the psychological effects of timeless, enduring sex differences in reproductive investment seems implausible to many. A recent article in the Economist, for example, equated the marketing-inspired tradition of the diamond engagement ring with the strutting peacock's extravagant tail, an evolved courtship ritual that signals a man's resources and commitment. The journalist wrote that "greater equality for women might seem to render male-courtship displays redundant. But mating preferences evolved over millennia and will not change quickly."
ENVIRONMENTAL INFLUENCE
ALTHOUGH SEX certainly influences the brain, this argument overlooks the growing recognition in evolutionary biology that offspring do not just inherit genes. They also inherit a particular social and ecological environment that can play a critical role in the expression of adaptive traits. For example, adult male moths that hailed, as larvae, from a dense population develop particularly large testes. These enhanced organs stand the moths in good stead for engaging in intense copulatory competition against the many other males in the population. One would be forgiven for assuming that these generously sized gonads are a genetically determined adaptive trait. Yet adult male moths of the same species raised as larvae in a lower-density population instead develop larger wings and antennae, which are ideal for searching for widely dispersed females.
If the development of sex-linked physical characteristics can be influenced by the social environment, it stands to reason that sex-linked behavior can be, too. One striking example comes from the previously mentioned female katydids, which compete for the males that bring them both sperm and food, in line with the Bateman-Trivers principles. Remarkably, when their environment becomes rich with nutritious pollen, their competitive "nature" wanes.
The environment is similarly important for adaptive behavior in mammals. Research published starting in the late 1970s found that rat mothers care for male and female pups differently. The males get licked more than the females in the anogenital region because the mothers are attracted to the higher level of testosterone in male pups' urine. Intriguingly, the greater stimulation from this higher-intensity licking plays a part in the development of sex differences in parts of the brain involved in basic masculine mating behavior.
As University of Sydney philosopher of science Paul Griffiths has observed, we should not be surprised that environmental factors or experiences that reliably recur every generation should be incorporated as inputs into the developmental processes that bring about evolved traits.
In our own species, these developmental inputs include the rich cultural inheritance bestowed on every human newborn. And although social constructions of gender vary across time and place, all societies weight biological sex with heavy cultural meaning. Gender socialization starts at birth, and it would only make sense if the ruthless process of natural selection were to exploit it. It may well have been adaptive in our evolutionary past for males to take these and those risks or for females to avoid them. But when culture changes—creating a very different pattern of rewards, punishments, norms and consequences, compared with those in the past—so, too, will patterns of sex differences in behavior.
Thus, the Economist writer was not quite right in stating that human "mating preferences evolved over millennia and will not change quickly." True, they are unlikely to change as quickly as those of katydids, with a sprinkling of pollen (although we suspect that is not what was meant). There is usually nothing simple and quick about creating cultural shifts. But change certainly can, and certainly has, taken place over timescales shorter than millennia.
Take, for example, gender gaps in the importance men and women place on a partner's financial resources, attractiveness and chastity. The very quaintness of the term "chastity" to Western ears today compared with several decades ago speaks to rapid changes in cultural gender expectations. Cross-culturally, women and men from countries with greater gender equity are more similar in all these dimensions of partner preferences than those from countries with lower equity between the sexes, according to a 2012 study by Marcel Zentner and Klaudia Mitura, both then at the University of York in England. Research has also shown that in the U.S., men now place more importance on a female partner's financial prospects, education and intelligence—and care less about her culinary and housekeeping skills—than they did several decades ago. Meanwhile the cliché of the pitiable bluestocking spinster is a historical relic: although wealthier and better-educated women were once less likely to marry, now they are more likely to do so.
Could we, then, see the day when the world's finest art galleries display as much art by women as by men? We certainly shouldn't let Bateman's fruit flies tell us no.
IN BRIEF
The classic evolutionary explanation of behavioral differences between men and women holds that they reflect an evolved pattern seen across the animal kingdom. Many gender gaps, in this view, are thus "natural." But research has shown that many of the foundational assumptions of this account are wrong. Environmental factors, moreover, can play a key role in the development of evolved behaviors. Progressive cultural shifts do not "go against" nature but rewrite it.
MY ONE AND ONLY?
Although promiscuous mating has traditionally been viewed as a male trait that evolved to maximize male reproductive success, scientists have documented dozens of species across the animal kingdom in which females have more offspring when they mate with multiple males.
MORE TO EXPLORE
Sexual Selections: What We Can and Can't Learn about Sex from Animals. Marlene Zuk. University of California Press, 2002.
Rethinking Bateman's Principles: Challenging Persistent Myths of Sexually Reluctant Females and Promiscuous Males. Zuleyma Tang-Martínez in Journal of Sex Research, Vol. 53, Nos. 4—5, pages 532—559; 2016.
Inferior: How Science Got Women Wrong—And the New Research That's Rewriting the Story. Angela Saini. Beacon Press, 2017.
Testosterone Rex: Myths of Sex, Science, and Society. Cordelia Fine. W. W. Norton, 2017.
FROM OUR ARCHIVES
The New Social Darwinists. John Horgan; October 1995.
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Cordelia Fine is a professor of history and philosophy of science at the University of Melbourne and a Women's Leadership Institute Australia Fellow. Her most recent book is Testosterone Rex: Myths of Sex, Science, and Society (W. W. Norton, 2017).
Mark A. Elgar is a professor of evolutionary biology at the University of Melbourne. His research group focuses on questions about the evolutionary significance of cooperative and mating behaviors and studies the role of chemical and visual communication in facilitating those behaviors.
