Editor's note: Rob Brooks is Professor of Evolution and Director of the Evolution & Ecology Research Centre at the University of New South Wales in Sydney, Australia. He studies the evolution of sexual behavior in humans and other animals. His first book, Sex, Genes & Rock 'n' Roll: How Evolution has Shaped the Modern World recently won the Queensland Literary Award for Science Writing.
In the mid-19th Century, two devastating floods of the Yellow River, and the famine that followed, ravaged northeastern China.
Outlaw bands, known as nien, attracted young men in unprecedented numbers, aggregating into militias that wrought chaos on the troops and infrastructure of the ruling Qing. Although this Nien Rebellion and the larger Taiping rebellion in the South were eventually crushed, they devastated the Chinese economy and contributed to the ending of the Qing dynasty.
According to political scientists Valerie Hudson and Andrea den Boer, widespread female infanticide during the famine meant that as many as one quarter of young men in the region were "bare branches" -- as the Chinese expression goes -- unlikely ever to bear fruit. The Nien rebellion, they argued, was propelled by these surplus young men who had so few other prospects.
This story of the Nien Rebellion foreshadows one of the biggest issues that China will face in coming decades: the dramatic excess of young men.
A long history of son preference, particularly among the Han majority, has led to female infanticide and the neglect of daughters in some parts of China. But in recent decades, the spread of cheap ultrasound (enabling sex-determination in early-mid pregnancy) and easy access to abortion courtesy of the government's one-child policy, has led to the widespread abortion of female fetuses.
As a result, approximately 30 million more men than women will reach adulthood and enter China's mating market by 2020.