Most people think overpopulation is one of the worst dangers facing the globe. In fact, the opposite is true. As countries get richer, their populations age and their birthrates plummet. And this is not just a problem of rich countries: the developing world is also getting older fast. Falling birthrates might seem beneficial, but the economic and social price is too steep to pay. The right policies could help turn the tide, but only if enacted before it's too late.