Mao Zedong once said that women hold up half the sky, and until I moved to China I believed it. My mother, a missionary’s daughter with a decidedly agnostic bent, was the first to tell me that in the People’s Republic men and women were equal. She had spent her teenage years in Asia before returning to the United States to study Chinese history, and when she informed me about Mao’s famous fraction she probably took out a photo album and pointed to photos of sensible-looking women with hair cropped into practical bobs. I can’t remember. In any case, the lifestyle she chose for us drove the lesson home.

When divorce left my mother with two young children and a mortgage, she took a Chinese friend into our Minneapolis home as a roommate. Hongyu was also recently divorced, and she had a son, who with my brother and me made three. Both Hongyu and my mother soon started graduate school, and they devised a strategy that might today be called coparenting. Back then it was called making do. They were something of an odd couple; my mother was happiest when dancing in a new outfit to Marvin Gaye albums, while Hongyu—who had grown up in Inner Mongolia during the Cultural Revolution—bought her clothes secondhand and could make a chicken last a week. But while they weren’t life partners, they were partners in raising us, trading off cooking and child care and planning outings and vacations together.

We lived on discount real estate, in a small house adjacent to the Minneapolis-St. Paul airport. Several times a day airplanes roared overhead, cutting so close to the roof that they darkened the sky and rendered conver- sation all but impossible. Between the planes and three children stir-crazy from Minnesota winters, I am not sure how my mother and Hongyu ever managed to study. But what I remember most from that time is an impression of strength. In our house women held up all the sky—and took out the garbage.

That impression stayed with me as I grew up, started studying Chinese, and finally went to China. In the summer of 2000 I spent a few months in Beijing on a language course. I was twenty and in college and had seen very little of the world, but from what I could tell my childhood vision of gender equality was accurate. China had female tycoons, female scientists, female writers, and in some ways the lot of women—like that of men—was improving every day. The faces in the photos from my mother’s 1980s albums had projected a sort of grim hopefulness. Back then women were so proud to own refrigerators that they crocheted dust covers for them and placed the appliances in the living room. (Then too most Chinese apartments had kitchens so small that refrigerators did not fit anywhere else.) By 2000 women were zipping across Beijing in Audis, dining at fancy restaurants, and stopping in for coffee afterward at Starbucks.

But there were also signs of trouble. Midway through the summer our teachers took us on a field trip to a kindergarten. Probably the goal was to have us talk with the one subset of Chinese people who shared our limited power of expression. What I remember, though, is the school’s population. In the sea of tiny smiles that greeted us, boys outnumbered girls.

On the bus ride back to the university one of our instructors, an energetic, sturdy woman named Teacher Zhang, explained in slow, clearly enunciated Chinese. I couldn’t have known the word for “ultrasound,” which had been imported from the West so recently that it contained a piece of the Roman alphabet: B超. But somehow I understood: some women were going in for scans halfway through their pregnancies. If they discovered their fetus was female, they would abort.

I wish I could say that was my eureka moment, that I fast-forwarded to what it would mean for China as the boys in the kindergarten grew up—that I looked into the issue and realized boys were proliferating in India, Azerbaijan, Vietnam, South Korea, and Albania as well. But the truth is I didn’t imagine the sex ratio imbalance could endure. While ultrasound technology was modern, like many people at the time I thought that using it for something as crass as sex selection had to be temporary: one last instance of sexist traditions rearing their ugly head.
It was only after I moved to China to work as a journalist four years later that I started to dwell on the societal implications of a population with tens of millions more men than women. The scene from the kindergarten repeated itself again and again. Once I journeyed to a small city in Shandong province to write an article on the solar heating system being installed in a school, and I found myself in another classroom full of smiling boys. I was tempted to abandon the solar power article and interview the teachers about the school’s population. Being my mother’s child, and being Hongyu’s child, I didn’t understand it. But it was clear the sky was sagging.

* * *

For as long as they have counted births, demographers have noted that on average 105 boys are born for every 100 girls. This is our natural sex ratio at birth. The ratio can vary slightly in certain conditions and from one geographic region to the next. More boys are born after wars. More girls are born around the equator, for reasons we don’t yet understand. But in general the sex ratio at birth hovers around 105.

So is our population male-dominated from the start? To the contrary: that more boys are born is itself a form of balance, neatly making up for the fact that males are more likely to die young. That extra 5 percent of boy babies compensates, as the German statistician Johann Peter Süssmilch observed in 1741, “for the higher male losses due to the recklessness of boys, to exhaustion, to dangerous tasks, to war, to sailing and emigration, thus maintaining the balance between the sexes so that everyone can find a spouse at the appropriate time of marriage.” While today males are less likely to die from sailing, exhaustion, or migration, they still account for the majority of soldiers throughout the world. They also disproportionately expose themselves to threats like smoking—a man’s pursuit in many countries—or riding motorcycles without wearing a helmet. Boys outnumber girls at birth because men outnumber women in early deaths.

Süssmilch, who was also a priest, was an early proponent of intelligent design; he concluded this natural check was the work of a meticulous creator. (The book in which he put forth his theory was titled The Divine Order as Derived from Demography.) When Charles Darwin looked into the sex ratio at birth a century later, he intuited that a balanced number of males and females instead connected somehow to evolution. Trends in human populations, Darwin noted, paralleled those found in the animal world. But that raised a question: What then was the purpose of the intense battles for mates among many species? To witness “two males fighting for the possession of the female, or several male birds displaying their gorgeous plumage, and performing strange antics before an assembled body of females,” as Darwin wrote in The Descent of Man, it was clear that a fierce evolutionary competition was at work. This competition was perhaps most evident in the peacock’s feathers: the colorful plumes would make sense if, as a rule, the sex ratio were skewed. If peahens were generally scarce, the male birds’ adornment would be a feature they had developed over generations to boost their chances of passing on their genes. A balanced sex ratio meant even the ugliest and most pitiful peacock had hope of finding a peahen.

But after extensive correspondence with farmers, shepherds, and biologists—Darwin even dutifully tallied sex ratios among English racehorses—the naturalist determined most species were in fact balanced. “After investigating, as far as possible, the numerical proportion of the sexes,” he wrote, “I do not believe that any great inequality in number commonly exists.”

Darwin went back and forth on exactly how a balanced sex ratio could be reconciled with his theory of natural selection, coming very close to a solution in the first edition of The Descent of Man and then retracting it in the second edition. “I now see that the whole problem is so intricate that it is safer to leave its solution for the future,” he wrote. And yet the naturalist surmised that balanced sex ratios were somehow critical to species survival.

In 1930 the English scientist Ronald A. Fisher arrived at the explanation that had eluded Darwin. Fisher’s theory works like this in humans: if male births become less common, men have better mating prospects than women. People with an assumed genetic disposition to have boys then have an advantage in passing on their genes. Put more simply, parents of sons have more grandchildren than parents of daughters. As the overall sex ratio approaches equilibrium, however, the advantage of producing sons disappears, and the sex ratio at birth balances out.
(Unfortunately, this mechanism does not work on skewed sex ratios of the sort seen in Asia today.) Fisher was also an enthusiastic eugenicist who believed in sterilizing the “unfit.” With John Maynard Keynes, he was among the founding members of the Cambridge University Eugenics Society. But he enshrined in evolutionary biology the notion that sex ratios are naturally balanced. Today a 1:1 sex ratio is called “Fisherian.”

A balanced sex ratio is now considered healthy in most species, to the extent that conservation work often focuses on boosting the number of females. It isn’t just that females are the ones who bear offspring, though of course that matters. In mammals who spend years rearing their young a skewed sex ratio can quickly veer out of control. If females are scarce, males may kill a female’s existing offspring to maximize their chance at passing on their genes, inadvertently speeding up the species’ path toward extinction. When the sex ratio of a group of brown bears living in the French Pyrenees recently skewed male, conservationists recommended a relocation program aimed at bringing males closer to potential mates. As one scientist put it, “Male bears need more females.”

But when it comes to our own species, we are considerably less attentive. While evolution encourages a balanced sex ratio, our large brains have always worked against one. For as long as we have documented reproduction, we have also sought ways to control it.
The ancient Greeks believed that when it came to procreation men’s testicles had specific roles: the left testicle produced girls, while the right one yielded boys. Aristotle took this to its logical but painful conclusion, teaching that men should tie off their left testicle during intercourse if they wanted a son. Well into the eighteenth century European men continued to follow this line of thinking; some went so far as to cut off their left gonad. But Aristotle also believed a baby’s sex was determined by a number of other factors. Women, he advised, should help their suffering husbands by making an effort to “think male.” And he observed, based on interviews with farmers, that with livestock “more males are born if copulation takes place when north [rather] than when south winds are blowing.”

The Greeks were hardly alone in offering complicated prescriptions for sex control. The Talmud advised men to bring their wives to early orgasm in order to have a son, advice that may have ended up in more pregnancies but probably had little effect on the sex ratio. And Indian ayurvedic texts outlined practices for manipulating the sex of a fetus—once it was in the mother’s womb.

But it is only in the past three decades that we have been able to control a baby’s sex with certainty. Our new capabilities demand a reconsideration of Darwin’s work. What does it mean to tinker with one of evolution’s most fundamental balances? Do we have the hubris to assume that what disrupts the brown bear won’t affect us? We still don’t know the evolutionary effects of fundamentally altering the sex ratio at birth, but a cursory glance back at history suggests it is not a great idea to mess with something we don’t understand.

If anything, we are making more of a mess of our species than brown bears ever could of theirs. When I started thinking about this book, I pictured talking to parents, demographers, perhaps a few government officials. I did not imagine I was beginning a journey on which I’d encounter prostitutes and trafficked wives and mail-order brides, gun enthusiasts and militant nationalists and the proprietor of a Fight Club–like “anger bar,” geneticists and AIDS researchers—and a lone U.S. military contractor. I did not know it would take me back in time to the American Wild West and into the future to the 2047 India that now preoccupies fans of science fiction. I did not picture villages in poor countries where most women have been sold or villages in rich countries where most have been bought. I had only the vague idea that a sharp decrease in women could not be good for the human race. And on that point I was proven sadly correct….

Like Unnatural Selection on Facebook for a PDF of the complete prologue.