Republican presidential hopefuls tell us America doesn’t want to become Europe. Former Pennsylvania Sen. Rick Santorum’s hat trick in the heartland bears this out. Santorum’s brand of social conservatism underpins one of the most striking differences between the United States and Europe — its demographic exceptionalism. This exceptionalism can make continued U.S. leadership in the world a reality.

Americans alone among the citizens of developed countries are having enough children to replace themselves. Demographers do not know why Americans buck the trend toward sub-replacement fertility. Immigration only accounts for part of the boost, and values matter a lot. Optimism, a frontier spirit and especially the high rate of religious participation that fosters family life have been shown to correlate to higher fertility.

Larger families tend to populate the same U.S. regions where these conservative values are high, and families in these areas are more likely to send their sons and daughters into the service. This has in turn allowed the U.S. military to recruit and retain a fighting force younger, fitter and better educated than its general working-age population despite the adverse recruiting environment of fighting two wars.

By contrast, the high cost of paying for an aging population in a social democracy makes European soldiers so costly they were pulled off the front lines in Afghanistan. Britain has had to abandon an independent aircraft carrier fleet and nuclear weapons capability, pooling assets with France. Russia doesn’t have enough healthy 18-year-olds to meet recruitment goals because of decades of low birth rates and a public health crisis. Tactical nukes became the backstop to dwindling brigade elements in a military exercise in the Russian Far East.

Some say the United States should follow suit by making deep defense cuts now to pay for bloated entitlements and limp along with its allies into a geriatric world peace.

But fewer troops keeping the peace will make the world less secure. Coming shifts in the global pecking order will likely have destabilizing effects on great power equipoise. This will happen just when cooperation among the powers will be needed to diffuse tensions in the developing world, which will in turn be exacerbated by its own diverging demographic trends.

Because of its demographic advantage, Americans have a few more years to put their fiscal house in order before the kind of draconian defense cuts its allies have made become necessary. For example, while the rest of the developed world is losing its working-age population (Russia, Germany and Japan expect more than a 25 percent drop by 2050 and China has already witnessed a decline), the United States will see a 16 percent gain in the same period.

Underlying American exceptionalism abroad is its demographic exceptionalism at home. Where Mitt Romney understands the effects of American power, Santorum has tapped into its source. None of the candidates has connected the dots to unify the Republican Party’s social, fiscal and foreign policy conservatives, and this has made the Republican caucuses a wild ride. To win, the nominee needs to understand all the aspects of American exceptionalism — and tell us how he will seize the demographic advantage.

Susan Yoshihara is senior vice president at C-FAM, the Catholic Family & Human Rights Institute. She is co-editor of “Population Decline and the Remaking of Great Power Politics.”