There are over 160 million females “missing” from Asia’s population. That’s more than the entire female population of the United States. And gender imbalance—which is mainly the result of sex selective abortion—is no longer strictly an Asian problem. In Azerbaijan and Armenia, in Eastern Europe, and even among some groups in the United States, couples are making sure at least one of their children is a son. So many parents now select for boys that they have skewed the sex ratio at birth of the entire world.

How did this occur? Why are women and girls becoming scarce in Asia and Eastern Europe as those regions develop? And what will happen when the world’s extra boys grow up?

A finalist for the Pulitzer Prize and the Los Angeles Times Book Prize, Unnatural Selection: Choosing Boys Over Girls, and the Consequences of a World Full of Men addresses these questions and more—and reveals some unexpected answers. Drawing on extensive reporting in China, India, Vietnam, South Korea, Albania, and other countries, Science magazine correspondent Mara Hvistendahl weaves together the story behind the world’s “missing” women into a riveting narrative. The cast of Unnatural Selection includes everyone from prostitutes, mail-order brides, and militant nationalists to geneticists, activists, and AIDS researchers—along with the California fertility doctors hard at work selling the world’s parents on the latest form of sex selection. But perhaps the book’s most disturbing finding is that the gender imbalance is not, as has often been argued, simply the outgrowth of entrenched sexism. In fact the story of the world’s missing women traces, in part, to the United States.

Unnatural Selection has been translated into German, Korean, and Japanese.