Written By JONATHAN V. LAST

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... http://online.wsj.com/article/SB1000142405270230365740457636169116563136...