The tiny city-state of Singapore is getting smaller. And dangerously so. In the 1970s, when the average number of children per woman was six, Singapore worried about overpopulation, and it launched a "Stop at Two" campaign. That campaign worked so well that the government had to reverse itself. In 1987 it launched a "Have Three or More" policy, but that didn't work. So the government, which once rewarded parents who had themselves voluntarily sterilized, is now offering paid maternity leaves, thousands of dollars in baby bonuses, and tax breaks for nannies. And according to a Reuters report, the government has now "appointed a 'population czar' to reignite the embers of romance in a nation obsessed with cars, cash, and condominiums."

Nations, from the small to the great, are scratching their heads over falling birthrates. Both developed and developing countries face a demographic dive that threatens their future. Yesterday's fears of overpopulation now seem quaint, as nations realize they do not have enough younger workers to pay for the social benefits of older citizens. In The Empty Cradle, Phillip Longman reports that "no industrialized nation still produces enough children to sustain its population over time, or to prevent rapid population aging."

The United States is no longer in depopulation mode, but it too faces a benefits crisis as population aging threatens to create "50 Floridas." Longman reports that by 2050, the cost of Medicare and Medicaid alone "will consume a larger share of the nation's income than the entire federal government does today, including the growing cost of interest on the national debt." Higher taxes on younger workers won't make up the difference.

Nor will increased immigration make up the difference—in ...