Just before Christmas there was a lot of public concern about America’s declining birthrate, which closed out 2012 at its lowest point since 1920. But in trying to understand why American fertility is on the wane, it’s important to understand that fertility decline is a global phenomenon. Ninety-seven percent of the world’s population lives in countries with declining fertility rates. And as bad as America has it now, things could be worse. We could be Japan.

Japan’s fertility rate has been below—far below—the replacement level for decades. And the demographic chickens are finally coming home to roost. Low fertility rates cause the age structure of a society to shift so that you have more old people than young people. Which means that once your final above-replacement generation gets ready to shuffle off the mortal coil, you have a whole lot of people dying every year and not very many people in the age-cohorts where childbearing occurs.