

Dinodia/The Image Works

In the struggle for equality between the sexes, it keeps coming down to motherhood, doesn’t it? Consider a recent article by Hanna Rosin in The Atlantic. Rosin finds that nursing her infant is holding her back from the work she enjoys, despite her plan for a fully egalitarian marriage. “We were raised to expect that co-parenting was an attainable goal,” she laments, yet breast-feeding ties her, and not her husband, to their baby. She combs through research on the health benefits of breast-feeding for babies and makes a convincing case that they aren’t as strong as experts have insisted. So does she quit nursing? She does not—even though, she admits, “I’m not really sure why.”

Rosin is a thoughtful writer, which makes her bewilderment all the more puzzling. She is, after all, a mammal, a member of a species that evolved mammary glands; doesn’t it seem likely that this might have some impact on her experience of life? Of course, she is hardly alone in avoiding this conclusion. Evolutionary science has been nearly as vexing a subject for feminists as for rural Texas school boards. Feminists consider sexual identity a “social construct,” a human—or, to be more precise, a male—invention. Evolutionary scientists, on the other hand, believe that we have inborn physical and psychological traits that result from millennia of adaptations to our natural environment. Where feminists see society, evolutionists see nature.

Especially galling to feminists has been the field of evolutionary psychology, which proposes that evolution has fundamentally shaped human sexual and reproductive behavior—behavior that often seems to conform to the worst stereotypes. So New York Times science writer Natalie Angier refers to evolutionary explanations of why older men prefer younger women as “just so stories” told by “evo-psychos.” Recently in Newsweek, Sharon Begley critiqued evolutionary psychology–inspired apologias for poor behavior by the likes of John Edwards and Eliot Spitzer before gladly pronouncing the field dead as a dinosaur.

Begley is right that pop evolutionary psychology often bears about the same relation to science as an episode of The Flintstones does to the Pleistocene era. But she’s wrong about the field’s being on its way out. If anything, recent findings in primatology, neuroscience, and genetics have given evo-psych new life. Scientists in these fields, many of them women, have lent support to some deeply controversial ideas about differences between the sexes. Among the most troubling for women like Rosin is that their inner conflict between child rearing and independence may be a battle between two powerful evolutionary forces.

If there’s one part of evolutionary thinking that spells bad news for the feminist worldview, it is parental-investment theory, an idea originally proposed by Harvard professor Robert Trivers. Trivers was attempting to clarify Darwin’s theory of sexual selection, which went something like this: females of most species are more particular about their mates than males are. That means males must compete for female attention; hence the colorful tails of peacocks and the lovely songs of many male birds.

But why should females be pickier than males? In 1972, Trivers offered an answer. He observed that in just about every species, it’s the females that gestate the young. When an animal nurses its offspring, as is the case with mammals, that’s part of the female job description, too. It is also mostly females who feed and guard the kids. In fact, females do nearly everything that increases the survival, and eventual reproductive success, of their offspring. Trivers concluded, logically enough, that as the sex with so much more at stake in gestating and feeding, females would take a stronger interest in their young. Females, as he put it, “invest” more than males—and that includes being cautious about their sexual partners, the fathers of their offspring.

Scientists, including the large number of female experts who have entered the field over the past 35 years, have had plenty of time to prod Trivers’s parental-investment theory for weaknesses. They’ve found a few. For one thing, females don’t always act as invested in their offspring as Trivers’s theory says that they should be. In conditions of extreme scarcity, for instance, mothers abandon or even kill their infants. Biologist Marlene Zuk has also noted that monkeys deprived of maternal care show little interest in their young. For another, males are sometimes big investors. In Why Sex Matters, the zoologist Bobbi Low cites a number of species, including the notorious praying mantis, whose males not only invest but make the final sacrifice. After mating, the female bites the male’s head off. (Researchers maintain that’s only if she is hungry.)

It’s also the case that high-investing females are sometimes promiscuous, though in accordance with Trivers’s general thesis, only when it benefits their young. One of Trivers’s students, the primatologist Sarah Blaffer Hrdy, discovered that female Hanuman langurs of India will mate with a large number of outside males. Male langurs often enter the troop and kill nursing infants, evidently hoping that the now-childless females will be available to mate with them. By “sleeping around,” a female langur engages in what is effectively a clever counterstrategy. She raises nature’s equivalent of self-conscious doubt in aggressive males—What if this is my baby, carrying my genes?—making them less likely to kill the kids.

As a theory, parental investment is more than an addendum to Darwin’s original sexual selection. Parental—which almost always means maternal—investment governs mating and reproduction. The profound female connection to her offspring is the Rosetta stone of female sexual behavior. Evolutionary scientists disagree on some details of sexual selection, but for all its uncomfortable implications, parental investment has been largely free of dissenters. Just about all scientists have signed on to Trivers’s basic template that in nature, females almost always do the kids.

The notion that females are more highly invested in their children than males is being confirmed by findings in biochemistry and neuroscience, as these disciplines clarify the role of hormones—particularly testosterone and oxytocin—in sexual and reproductive behavior. Like the male sex hormone testosterone, oxytocin is produced in the hypothalamus. But in most other respects, it is the anti-testosterone. Instead of fueling aggression, it promotes attachment, reduces fear, and leads to feelings of pleasure and well-being. Testosterone appears in males at far higher levels than in females; oxytocin, on the other hand, is more prevalent in females. Women have many more oxytocin receptors in their brains than men do, and those receptors rev up during orgasm, childbirth, and breast-feeding—signaling that at a biological level, the boundaries most of us take as axiomatic between sexual pleasure, reproduction, and mothering are not all that clear. Hrdy goes so far as to conclude that “the ‘afterglow’ from climax is an ancient ‘maternal’ rather than sexual response.” In females, in other words, the maternal urge shapes the sexual urge.

Oxytocin may explain what Katie Roiphe, a journalism professor at New York University, meant when, in a recent essay, she described an “addiction” to her newborn baby that left her indifferent to work. Many female readers were perturbed: Roiphe was feeding the cult of motherhood, they said; maternal love is neither an interesting nor a useful subject for women today.

But surely it’s worth understanding the natural forces at work in our everyday experience. Evolutionary psychology tells us that our neural systems evolved in ways that enhanced survival. Maternal attachment was essential to that project, since babies without mothers were at much higher risk of death. Evolution selected for women like Roiphe and Rosin who wanted to hold and nurse their infants. Since women with more oxytocin receptors were most successful at reproducing, they tended to pass down the genes that ensured the same hormonal sensitivity in their offspring. Conversely, for survival, infants needed to attach to their mothers; not surprisingly, oxytocin is transmitted to babies in breast milk. (Researchers are pursuing evidence that autistics, who have trouble attaching emotionally to others, are abnormally low in oxytocin.)

There may or may not be a “maternal instinct”—like many female academics, Hrdy objects to the term—but there is a hormone that amounts to almost the same thing. It inclines females to feed, cuddle, and fuss over their young, and leaves men at peace.

If that were evolutionary psychology’s whole story about women, then its experts would be proclaiming patriarchy as our destiny, which they don’t tend to do. In fact, as neuroscientists and geneticists piece together the human brain’s evolution, it’s becoming clear that, if it’s natural for a woman to go crazy over her babies, it’s also natural for a woman to run the State Department. The same human female brain that’s primed with oxytocin is, like the male brain, a fantastically complex machine, capable of reasoning, innovative problem solving, and maneuvering through hugely varied social environments—whether the PTA, a corporate headquarters, or Congress.

Human beings are called Homo sapiens for good reason. We evolved brains proportionately bigger than any other animal’s, and when it comes to the gray stuff, size matters. The human cranial space is taken up by a large, densely wired frontal cortex, which allows us to create sentences and paragraphs, think abstractly, and plan for tomorrow’s meeting or next year’s vacation, all cognitive activities far beyond the capacity of any animal. The cortex is also the driver for human culture. When you ponder a picture of the Taj Mahal or attend a production of A Midsummer Night’s Dream, thank the evolution of the human brain. In a related way, the frontal cortex gives humans, unlike animals, the potential to control urges from the limbic system, whether for the second piece of red velvet cake or for the brunette behind the Starbucks counter. Next time you go to the zoo, look at the chimp’s low, recessed brow. Then check out your own bulging pate. That, clever reader, is your frontal cortex.

The frontal cortex is the reason that sexual selection does not provide the rigid behavioral script for humans that it does for animals. The chimpanzee brain gives the mating male and reproducing female no choice about how to go about the business. It’s humans who have invented everything from love marriages to purdah, from bordellos to nunneries.

Evolutionary psychologists are sometimes accused of not giving proper due to the flexibility of the human brain. In her recent book Mothers and Others, for instance, Hrdy argues that just as animal males don’t tend to their infants, so human fathers can’t be expected to hang around for the long run. But at their best, scientists are apt to describe the brain as chemically and neurologically predisposed to certain behaviors—nurturing babies in the case of women, for instance—while capable of adapting these behaviors to enormously varied environments. Sometimes those environments even change the brain’s chemistry, a process that the writer Matt Ridley calls “nature via nurture.” When Hrdy presumes the fecklessness of men, she underestimates the environmental pressure of social norms. The human record suggests that social norms, especially the universal one of marriage, can reinforce fathers’ ties to their children, which in turn might even become part of the male neural architecture. Recently, neuroscientists have even discovered evidence that married men’s testosterone levels fall at the birth of their baby.

Similarly, females adapt to complex societies that put a premium on abstract thinking and organizational skills. But they can do so only when the environmental conditions are right, as in the developed world today. Once again, we can thank Mother Nature—or, more precisely, the frontal cortex. Using our noggins, human beings have transformed the environment in which they mate and reproduce, expanding opportunities for females to employ their three pounds of brain muscle. Most people would point to the invention of the birth-control pill as one of the key moments in the cultural transformation that culminated in this contemporary ecosystem. But other human inventions were vital as well.

Two recent papers by economists tell some of the story. In one, a working paper for the National Bureau of Economic Research, Stefania Albanesi and Claudia Olivetti show how child care was only one of the reasons women were once limited to the home; illness and injury caused by childbirth closed doors, too. “Consider a typical woman born around 1900,” the authors write. “She married at 21 and gave birth to more than three live children between age 23 and 33. The high fetal mortality rate implied an even greater number of pregnancies, so that she would be pregnant for 36 percent of this time. Health risks in connection to pregnancy and childbirth were severe. Septicemia, toxaemia, hemorrhages and obstructed labor could lead to prolonged physical disability and, in the extreme, death.” It wasn’t just the Pill, then; antibiotics, blood banks, improvements in prenatal and obstetric care, and the mass production of safe baby formula fundamentally altered the human environment in ways that laid the foundation for contemporary women’s achievement.

Machinery invented by the brainy Homo sapiens also revolutionized the female lot. Until 1900, the vast majority of people in the Western world lived in conditions much like those in sub-Saharan Africa and parts of the Middle East today. Few had access to electricity; only about a quarter of all American households had running water. In this environment, American women did what women tied to their domiciles with three-plus children have always done: cooking, making and cleaning clothes, hauling water, and the like. But by the mid-twentieth century, human innovation had considerably lightened those essential household tasks. Using U.S. Census data, University of Montreal economist Emanuela Cardia has shown how home technology, including appliances and bathroom plumbing, played a significant role in moving women into the labor force.

And so in the twentieth century, the big-brained female—Femina sapiens, if you will—found herself living in an utterly reshaped habitat, free from sepsis, unplanned children, sewing, bread baking, and arduous trips to the well. She was ready to use her brain in new ways, not coincidentally at a time when the intellectually gratifying jobs of an advanced economy were becoming more plentiful. It’s a neat coincidence that women wrote both of the papers just discussed. Men invented the antibiotic and the washing machine; today, women in economics departments calculate the benefits of these discoveries for their sex. Better yet, they themselves can make future discoveries in labs and R&D departments. (Whether their cognitive makeup makes them less inclined to take up such scientific tasks remains a matter of intense disagreement among scientists.) In a 2007 paper, economists Justin Wolfers and Betsy Stevenson noted that young women attach greater importance than in the past to “being successful in my line of work,” “being able to find steady work,” “making a contribution to society,” and “being a leader in my community.” Given both their neural architecture and the new cultural ecology, you could hardly call this unnatural.

But you could call it problematic, as new mothers like Rosin and Roiphe are rediscovering. The contemporary woman is in a bind. Her brain (crudely put, her hypothalamus) is at war with her brain (equally crudely, her frontal cortex). She wants two things at once, and they are often contradictory. Complicating her life further, the frontal cortex of her own children will take forever—some say over 20 years—to develop fully. That means that she faces many more years with dependent offspring than females of other species who can’t write briefs or paint canvases, yet have nothing else to do with their time. It’s unfair!

And here’s another bitter pill for women: more complex societies like our own require a more highly developed frontal cortex. To thrive in today’s complex economy, children have to undergo many years of intensive training. The final irony for Femina sapiens is that she may well find herself sacrificing some potential achievements to raise the child who goes on to invent a device that makes life richer for future generations of women.

If human society can sometimes reconfigure biology—by curing polio and increasing athletic stamina, for example—could it reconfigure sexual selection so that fathers and mothers made equal investments in their young children? We don’t have much evidence for thinking so. Until the mid-1990s, Swedish parents got nine months of leave after the birth of a child, and in theory, either mothers or fathers could use it; in practice, it might as well have been called maternal leave. Frustrated that so few fathers took advantage of the policy, the Swedish government changed the way it worked: fathers would now get a month of leave (or two, as of 2002) that they weren’t allowed to transfer to their wives. The results were just as evolutionary psychologists would predict. By 2004, only 20 percent of fathers were taking the two months. By contrast, a large majority of mothers made full use of their leave. Iceland launched a similar effort to equalize parental investment; fathers there are doing more, but nowhere near as much as mothers.

The predicament of Femina sapiens poses immense practical challenges that should be a subject of public debate. How do we make it easier for working women who want more time to invest in their young children to work part time, or to return to their jobs after an extended leave? What is the proper role of government in all this?

Meantime, though, it might help if women understood their predicament philosophically. Since its beginnings, many people have objected to evolutionary theory as reductive and dismissive of humanity’s special place in the cosmos. I see it rather differently. Like his near-contemporary Freud, Darwin shows us to be profoundly mysterious to ourselves. We humans live on many levels: some are unique to us, but others connect us to our primordial ancestors and, indeed, to the natural world itself. “Taking care of the baby—physical, draining, exhilarating—is more like farming,” writes Roiphe, “following the rhythms of the earth, getting up at dawn, watching the corn flush in the sunrise. It is not at all like writing.” There is something thrilling in the mystery and embeddedness of this experience.

“There is a grandeur in this view of life,” Darwin wrote in a sentence much quoted during this, his 200th anniversary year. “From so simple a beginning the endless forms most beautiful and most wonderful have been, and are being evolved.” And so, too, Femina sapiens.