This interesting 2 minute video claims that a rising population without enough economic growth to provide jobs for everyone leads to civil unrest because of too many men with nothing to do.
The talking heads in this video say China, India, and Africa are particularly at risk – China especially because parents abort female fetuses more often, creating 20 million more men. (Chinese men are expected to care for their elderly parents, but women are not. In a country with no Social Security or 401(k) programs a son means survival once parents are too old to work.)
Much of the argument rests on correlation being causation, however.