The scene from the preschool in the Chinese county of Suinin, approximately halfway between Beijing and Shanghai, could have been an advertisement for cuteness. Two lines of children walk out of school at the end of the day, led by their beaming teachers towards a cluster of expectant parents. Mara Hvistendahl witnessed this scene but when she did, she noticed a strange phenomenon: there were nearly twice as many boys in the two lines as girls.
The situation in Suinin is hardly unique. Across the developing world, from China to India to Eastern Europe to the Middle East, sex ratios - the ratio of boy babies to girl babies - are becoming increasingly skewed. The normal human ratio is around 105 boys for every 100 girls, a natural evolutionary ratio that takes into account the fact that more boys tend to die before reaching adulthood. But in China today, the ratio is 121 boys for every 100 girls; in India the ratio is 112 boys for every 100 girls; in Tunisia the official ratio is 107 boys per 100 girls, although the real figure is believed to be much higher.
In her thorough and compelling new book, Unnatural Selection, Hvistendahl explains why these trends will have far-reaching effects. She argues that the sex imbalance could prove devastating to social stability across the developing world, sparking crime, human trafficking, and - if history is any guide - even war.