Rob Brooks, professor of evolution at New South Wales University in Australia, looks to the surplus of men over women in China as a potential threat to social stability. From CNN:
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.
[…] Many factors contribute to the number of men who will never find a mate. Economic inequality, for one, leaves a great many poor young men unable to attract a wife. When a society allows powerful men to take several wives, too few women remain for many poor men to take even a single wife. But most dramatically of all, male-biased sex ratios consign the excess men to never having a family of their own.
[…] China is already feeling the effects of so many bare branches. The economist Lena Edlund estimates that every one percent increase in the sex ratio results in a six percent increase in the rates of violent and property crime. In addition, the parts of China with the most male-biased sex ratios are experiencing a variety of other maladies, all tied to the presence of too many young men. Gambling, alcohol and drug abuse, kidnapping and trafficking of women are rising steeply in China.
See more on China’s gender imbalance via CDT.