W. Bradford Wilcox, who consults on U.S. demographics, is the director of the National Marriage Project at the University of Virginia.

The long-term fortunes of the modern economy depend in part on the strength and sustainability of the family, both in relation to fertility trends and to marriage trends. This basic, but often overlooked, principle is now at work in the current global economic crisis.

The decline of marriage and fertility is one factor in the global economic crisis.

That is, one reason that some of the world’s leading economies — from Japan to Italy to Spain to the euro zone as a whole — are facing fiscal challenges is that their fertility rates have been below replacement levels (2.1 children per woman) for decades. Persistent sub-replacement fertility eventually translates into fewer workers relative to retirees, which puts tremendous strains on public coffers and the economy as a whole. Indeed, one recent study finds that almost half of the recent run-up in public debt in the West can be attributed to rapid aging over the last two decades.

Even China may see its sky-high growth “come down to earth in the next few decades as its work force shrinks” because of its one-child policy, as Carlos Cavallé and I argued in a recent report, The Sustainable Demographic Dividend. By contrast, a recent Rand study suggests that “India will have more favorable demographics than China” over the next few decades, insofar as its work force is poised to grow. In fact, the Rand study suggests that India may be able to use this demographic advantage to outpace China’s economic growth rates by the end of the century.

Finally, it’s not just fertility that matters; it’s also marriage. At least in the West, children are more likely to acquire the human and social capital they need to thrive in the modern economy when they are raised in an intact, married family. In the U.S., for instance, children are more likely to graduate from high school, complete college and be gainfully employed as young adults if they were raised in an intact, married family.

And around the globe, men are more likely to give their work their fullest effort and attention when they are married; this is one reason men worldwide enjoy “marriage premiums” in their income, ranging from about 14 percent (Mexico) to 19 percent (United States) to 35 percent (Russia). So, at least when it comes to men, research suggests that marriage has important implications for worker productivity.

The bottom-line message is that what happens in the home does not stay at home; rather, the size of families, and their stability and quality, has important implications for the health of the global economy.