In the eternally recurring debates about whether some rival great power will knock the United States off its global perch, there has always been one excellent reason to bet on a second American century: We have more babies than the competition.

It’s a near-universal law that modernity reduces fertility. But compared with the swiftly aging nations of East Asia and Western Europe, the American birthrate has proved consistently resilient, hovering around the level required to keep a population stable or growing over the long run. …

If, that is, our dynamism persists. But that’s no longer a sure thing. American fertility plunged with the stock market in 2008, and it hasn’t recovered. Last week, the Pew Research Center reported that U.S. birthrates hit the lowest rate ever recorded in 2011, with just 63 births per 1,000 women of childbearing age. (The rate was 71 per 1,000 in 1990.) For the first time in recent memory, Americans are having fewer babies than the French or British. …

But deeper forces than the financial crisis may keep American fertility rates depressed. Foreign-born birthrates will probably gradually recover from their current nadir, but with fertility in decline across Mexico and Latin America, it isn’t clear that the United States can continue to rely heavily on immigrant birthrates to help drive population growth.