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.

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.