Mara Hvistendahl is worried about girls. Not in any political, moral or cultural sense but as an existential matter. She is right to be. In China, India and numerous other countries (both developing and developed), there are many more men than women, the result of systematic campaigns against baby girls. In "Unnatural Selection," Ms. Hvistendahl reports on this gender imbalance: what it is, how it came to be and what it means for the future.

In nature, 105 boys are born for every 100 girls. This ratio is biologically ironclad. Between 104 and 106 is the normal range, and that's as far as the natural window goes. Any other number is the result of unnatural events.

Yet today in India there are 112 boys born for every 100 girls. In China, the number is 121—though plenty of Chinese towns are over the 150 mark. China's and India's populations are mammoth enough that their outlying sex ratios have skewed the global average to a biologically impossible 107. But the imbalance is not only in Asia. Azerbaijan stands at 115, Georgia at 118 and Armenia at 120.

What is causing the skewed ratio: abortion. If the male number in the sex ratio is above 106, it means that couples are having abortions when they find out the mother is carrying a girl. By Ms. Hvistendahl's counting, there have been so many sex-selective abortions in the past three decades that 163 million girls, who by biological averages should have been born, are missing from the world. Moral horror aside, this is likely to be of very large consequence.

'No. 23' (2005-06), a painting by Ma Liuming. Ma Liuming/Sotheby's

In the mid-1970s, amniocentesis, which reveals the sex of a baby in utero, became available in developing countries. Originally meant to test for fetal abnormalities, by the 1980s it was known as the "sex test" in India and other places where parents put a premium on sons. When amnio was replaced by the cheaper and less invasive ultrasound, it meant that most couples who wanted a baby boy could know ahead of time if they were going to have one and, if they were not, do something about it. "Better 500 rupees now than 5,000 later," reads one ad put out by an Indian clinic, a reference to the price of a sex test versus the cost of a dowry.

But oddly enough, Ms. Hvistendahl notes, it is usually a country's rich, not its poor, who lead the way in choosing against girls. "Sex selection typically starts with the urban, well-educated stratum of society," she writes. "Elites are the first to gain access to a new technology, whether MRI scanners, smart phones—or ultrasound machines." The behavior of elites then filters down until it becomes part of the broader culture. Even more unexpectedly, the decision to abort baby girls is usually made by women—either by the mother or, sometimes, the mother-in-law.

If you peer hard enough at the data, you can actually see parents demanding boys. Take South Korea. In 1989, the sex ratio for first births there was 104 boys for every 100 girls—perfectly normal. But couples who had a girl became increasingly desperate to acquire a boy. For second births, the male number climbed to 113; for third, to 185. Among fourth-born children, it was a mind-boggling 209. Even more alarming is that people maintain their cultural assumptions even in the diaspora; research shows a similar birth-preference pattern among couples of Chinese, Indian and Korean descent right here in America.

Unnatural Selection: Choosing Boys Over Girls and the Consequences of a World Full of Men By Mara Hvistendahl

PublicAffairs, 314 pages, $26.99

Ms. Hvistendahl argues that such imbalances are portents of Very Bad Things to come. "Historically, societies in which men substantially outnumber women are not nice places to live," she writes. "Often they are unstable. Sometimes they are violent." As examples she notes that high sex ratios were at play as far back as the fourth century B.C. in Athens—a particularly bloody time in Greek history—and during China's Taiping Rebellion in the mid-19th century. (Both eras featured widespread female infanticide.) She also notes that the dearth of women along the frontier in the American West probably had a lot to do with its being wild. In 1870, for instance, the sex ratio west of the Mississippi was 125 to 100. In California it was 166 to 100. In Nevada it was 320. In western Kansas, it was 768.

There is indeed compelling evidence of a link between sex ratios and violence. High sex ratios mean that a society is going to have "surplus men"—that is, men with no hope of marrying because there are not enough women. Such men accumulate in the lower classes, where risks of violence are already elevated. And unmarried men with limited incomes tend to make trouble. In Chinese provinces where the sex ratio has spiked, a crime wave has followed. Today in India, the best predictor of violence and crime for any given area is not income but sex ratio.

A high level of male births has other, far-reaching, effects. It becomes harder to secure a bride, and men can find themselves buying or bidding for them. This, Ms. Hvistendahl notes, contributes to China's astronomical household savings rate; parents know they must save up in order to secure brides for their sons. (An ironic reflection of the Indian ad campaigns suggesting parents save money by aborting girls.) This savings rate, in turn, drives the Chinese demand for U.S. Treasury bills.

And to beat the "marriage squeeze" caused by skewed sex ratios, men in wealthier imbalanced countries poach women from poorer ones. Ms. Hvistendahl reports from Vietnam, where the mail-order-bride business is booming thanks to the demand for women in China. Prostitution booms, too—and not the sex-positive kind that Western feminists are so fond of.

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The economist Gary Becker has noted that when women become scarce, their value increases, and he sees this as a positive development. But as Ms. Hvistendahl demonstrates, "this assessment is true only in the crudest sense." A 17-year-old girl in a developing country is in no position to capture her own value. Instead, a young woman may well become chattel, providing income either for their families or for pimps. As Columbia economics professor Lena Edlund observes: "The greatest danger associated with prenatal sex determination is the propagation of a female underclass," that a small but still significant group of the world's women will end up being stolen or sold from their homes and forced into prostitution or marriage.

All of this may sound dry, but Ms. Hvistendahl is a first-rate reporter and has filled "Unnatural Selection" with gripping details. She has interviewed demographers and doctors from Paris to Mumbai. She spends a devastating chapter talking with Paul Ehrlich, the man who mainstreamed overpopulation hysteria in 1968 with "The Population Bomb"—and who still seems to think that getting rid of girls is a capital idea (in part because it will keep families from having more and more children until they get a boy). In another chapter she speaks with Geert Jan Olsder, an obscure Dutch mathematician who, by an accident of history, contributed to the formation of China's "One Child" policy when he met a Chinese scientist in 1975. Later she visits the Nanjing headquarters of the "Patriot Club," an organization of Chinese surplus men who plot war games and play at mock combat.

Ms. Hvistendahl also dredges up plenty of unpleasant documents from Western actors like the Ford Foundation, the United Nations and Planned Parenthood, showing how they pushed sex-selective abortion as a means of controlling population growth. In 1976, for instance, the medical director of the International Planned Parenthood Federation, Malcom Potts, wrote that, when it came to developing nations, abortion was even better than birth control: "Early abortion is safe, effective, cheap and potentially the easiest method to administer."

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The following year another Planned Parenthood official celebrated China's coercive methods of family planning, noting that "persuasion and motivation [are] very effective in a society in which social sanctions can be applied against those who fail to cooperate in the construction of the socialist state." As early as 1969, the Population Council's Sheldon Segal was publicly proclaiming the benefits of sex-selective abortion as a means of combating the "population bomb" in the East. Overall Ms. Hvistendahl paints a detailed picture of Western Malthusians pushing a set of terrible policy prescriptions in an effort to road-test solutions to a problem that never actually manifested itself.

There is so much to recommend in "Unnatural Selection" that it's sad to report that Ms. Hvistendahl often displays an unbecoming political provincialism. She begins the book with an approving quote about gender equality from Mao Zedong and carries right along from there. Her desire to fault the West is so ingrained that she criticizes the British Empire's efforts to stamp out the practice of killing newborn girls in India because "they did so paternalistically, as tyrannical fathers." She says that the reason surplus men in the American West didn't take Native American women as brides was that "their particular Anglo-Saxon breed of racism precluded intermixing." (Through most of human history distinct racial and ethnic groups have only reluctantly intermarried; that she attributes this reluctance to a specific breed of "racism" says less about the American past than about her own biases.) When she writes that a certain idea dates "all the way back to the West's predominant creation myth," she means the Bible.

Ms. Hvistendahl is particularly worried that the "right wing" or the "Christian right"—as she labels those whose politics differ from her own—will use sex-selective abortion as part of a wider war on abortion itself. She believes that something must be done about the purposeful aborting of female babies or it could lead to "feminists' worst nightmare: a ban on all abortions."

It is telling that Ms. Hvistendahl identifies a ban on abortion—and not the killing of tens of millions of unborn girls—as the "worst nightmare" of feminism. Even though 163 million girls have been denied life solely because of their gender, she can't help seeing the problem through the lens of an American political issue. Yet, while she is not willing to say that something has gone terribly wrong with the pro-abortion movement, she does recognize that two ideas are coming into conflict: "After decades of fighting for a woman's right to choose the outcome of her own pregnancy, it is difficult to turn around and point out that women are abusing that right."

Late in "Unnatural Selection," Ms. Hvistendahl makes some suggestions as to how such "abuse" might be curbed without infringing on a woman's right to have an abortion. In attempting to serve these two diametrically opposed ideas, she proposes banning the common practice of revealing the sex of a baby to parents during ultrasound testing. And not just ban it, but have rigorous government enforcement, which would include nationwide sting operations designed to send doctors and ultrasound techs and nurses who reveal the sex of babies to jail. Beyond the police surveillance of obstetrics facilities, doctors would be required to "investigate women carrying female fetuses more thoroughly" when they request abortions, in order to ensure that their motives are not illegal.

Such a regime borders on the absurd. It is neither feasible nor tolerable—nor efficacious: Sex determination has been against the law in both China and India for years, to no effect. I suspect that Ms. Hvistendahl's counter-argument would be that China and India do not enforce their laws rigorously enough.

Despite the author's intentions, "Unnatural Selection" might be one of the most consequential books ever written in the campaign against abortion. It is aimed, like a heat-seeking missile, against the entire intellectual framework of "choice." For if "choice" is the moral imperative guiding abortion, then there is no way to take a stand against "gendercide." Aborting a baby because she is a girl is no different from aborting a baby because she has Down syndrome or because the mother's "mental health" requires it. Choice is choice. One Indian abortionist tells Ms. Hvistendahl: "I have patients who come and say 'I want to abort because if this baby is born it will be a Gemini, but I want a Libra.' "

This is where choice leads. This is where choice has already led. Ms. Hvistendahl may wish the matter otherwise, but there are only two alternatives: Restrict abortion or accept the slaughter of millions of baby girls and the calamities that are likely to come with it.

—Mr. Last is a senior writer at the Weekly Standard.