Conservatives like to think that if we could just provide the right tax incentives for childbearing, then Americans might go back to having children the way they did 40 years ago. Liberals like to think that if we would just be more like France—offer state-run day care and other programs so women wouldn’t have to choose between working and motherhood—it would solve the problem. But the evidence suggests that neither path offers more than marginal gains. France, for example, hasn’t been able to stay at the replacement rate, even with all its day-care spending.

Which leaves us with outsourcing our fertility. We’ve received a massive influx of immigrants from south of the border since the late 1970s. Immigration has kept America from careening over the demographic cliff. Today, there are roughly 38 million people in the U.S. who were born elsewhere. (Two-thirds of them are here legally.) To put that in perspective, consider that just four million babies are born annually in the U.S.

If you strip these immigrants—and their relatively high fertility rates—from our population profile, America suddenly looks an awful lot like continental Europe, which has a fertility rate of 1.5., if not quite as demographically terminal as Japan.

Relying on immigration to prop up our fertility rate also presents several problems, the most important of which is that it’s unlikely to last. Historically, countries with fertility rates below replacement level start to face their own labor shortages, and they send fewer people abroad. In Latin America, the rates of fertility decline are even more extreme than in the U.S. Many countries in South America are already below replacement level, and they send very few immigrants our way. And every other country in Central and South America is on a steep dive toward the replacement line.