In 1990, the economist Amartya Sen published an essay in The New York Review of Books with a bombshell title: “More Than 100 Million Women Are Missing.” His subject was the wildly off-kilter sex ratios in India, China and elsewhere in the developing world. To explain the numbers, Sen invoked the “neglect” of third-world women, citing disparities in health care, nutrition and education. He also noted that under China’s one-child policy, “some evidence exists of female infanticide.”

The essay did not mention abortion.

Twenty years later, the number of “missing” women has risen to more than 160 million, and a journalist named Mara Hvistendahl has given us a much more complete picture of what’s happened. Her book is called “Unnatural Selection: Choosing Boys Over Girls, and the Consequences of a World Full of Men.” As the title suggests, Hvistendahl argues that most of the missing females weren’t victims of neglect. They were selected out of existence, by ultrasound technology and second-trimester abortion.

The spread of sex-selective abortion is often framed as a simple case of modern science being abused by patriarchal, misogynistic cultures. Patriarchy is certainly part of the story, but as Hvistendahl points out, the reality is more complicated — and more depressing.

Thus far, female empowerment often seems to have led to more sex selection, not less. In many communities, she writes, “women use their increased autonomy to select for sons,” because male offspring bring higher social status. In countries like India, sex selection began in “the urban, well-educated stratum of society,” before spreading down the income ladder.