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.

America’s demographic edge has a variety of sources: our famous religiosity, our vast interior and wide-open spaces, our willingness to welcome immigrants (who tend to have higher birthrates than the native-born).

And it clearly is an edge. Today’s babies are tomorrow’s taxpayers, and relatively youthful populations speed economic growth and keep spending commitments affordable. Thanks to our demographic dynamism, the America of 50 years hence may not only have more workers per retiree than countries like Japan, but also more than emerging powers like China.

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. A week ago, 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.

The plunge might be temporary. American fertility plummeted during the Great Depression, and more recent downturns have produced modest dips as well. This time, the birthrate has fallen fastest among foreign-born Americans, and particularly among Hispanics. Many people may simply be postponing childbearing until better times return.

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.

Among the native-born working class, meanwhile, there was a retreat from child rearing even before the Great Recession hit. For Americans without college degrees, economic instability and a shortage of marriageable men seem to be furthering two trends in tandem: more women are having children out of wedlock, and fewer are raising families at all.

Finally, there’s been a broader cultural shift away from a child-centric understanding of romance and marriage. In 1990, 65 percent of Americans told Pew that children were “very important” to a successful marriage; in 2007, only 41 percent agreed.

Government’s power over fertility rates is limited, but not nonexistent. America has no real family policy to speak of at the moment, and the evidence from countries like Sweden and France suggests that reducing the ever-rising cost of having kids can help fertility rates rebound. Whether this means a more family-friendly tax code, a push for more flexible work hours, or an effort to reduce the cost of college, there’s clearly room for creative policy to make some difference.

More broadly, a more secure economic foundation beneath working-class Americans would presumably help promote childbearing as well. Stable families are crucial to prosperity and mobility, but the reverse is also true, and policies that made it easier to climb the economic ladder would make it easier to raise a family as well.

Beneath these policy debates, though, lie cultural forces that no legislator can really hope to change. The retreat from child rearing is, at some level, a symptom of late-modern exhaustion – a decadence that first arose in the West but now haunts rich societies around the globe. It’s a spirit that privileges the present over the future, chooses stagnation over innovation. It embraces the comforts and pleasures of modernity, while shrugging off the basic sacrifices that built our civilization in the first place.

Such decadence need not be permanent, but neither can it be undone by political willpower alone. It can only be reversed by the slow accumulation of individual choices, which is how all social and cultural recoveries are ultimately made.