In 2020, by some estimates, there will be 30 million more men than women on the mating market in China, leaving perhaps up to 15% of young men without mates.
Anthropologists have documented a consistent historical pattern: when the sex ratio skews in the direction of a smaller proportion of females, men become increasingly competitive, becoming more likely to engage in risky, short-term oriented behavior including gambling, drug abuse, and crime. This sort of pattern fits well with the rest of the biological world. Decades of work in behavioral ecology has shown that in species in which there is substantial variation in mating success among males, males compete especially fiercely.
The precise details of the route from a biased sex ratio to anti-social behavior in humans is not thoroughly understood, but one possible physiological link is that remaining unmarried increases levels of testosterone—often simply referred to as "T"—which in turn influences decision making and behavior.
Should all this T in China be a cause for worry?
The differences between societies that allow polygyny and those that don't are potentially illustrative. In societies with polygamy, there are, for obvious reasons, larger numbers of unmarried men than in societies that prohibit polygyny. These unmarried men compete for the remaining unmarried women, which includes a greater propensity to violence and engaging in more criminal behavior than their married counterparts. Indeed, cross-national research shows a consistent relationship between imbalanced sex ratios and rates of violent crime. The higher the fraction of unmarried men in a population, the greater the frequency of theft, fraud, rape, and murder. The size of these effects are non-trivial: Some estimates suggest marriage reduces the likelihood of criminal behavior by as much as one half.
Further, relatively poor unmarried men, historically, have formed associations with other unmarried men, using force to secure resources they otherwise would be unable to obtain.
While increasing crime and violence in Asian countries with imbalanced sex ratios is a reason to worry in itself, the issue is not only the potential victims of crimes that might occur because of the sex ratio imbalance. Evidence indicates that surpluses of unmarried young men have measurable economic effects, lowering per capita GDP.
China, of course, plays a crucial role in the modern heavily interconnected world economy, and is the largest or second largest trading partner for 78 countries. While many Americans worry about China "overtaking" the United States—as if economics were a zero sum game—the real danger stems from the ripples of a potential Chinese economic slowdown, whether from civil unrest or otherwise. Regional economies such as South Korea and Taiwan would no doubt be hard hit, but Europe and the United States would suffer disruptions of both supply and demand, which unpredictable, but possibly substantial, economic consequences.
The route from unmarried men to global economic meltdown is perhaps a bit indirect, but the importance of China in the world economy makes such threats to stability something to worry about.