Last week, the National Center for Health Statistics reported that America’s birthrate reached a historic low in 2017, falling to 60.2 births per 1,000 women of childbearing age. For a population in the developed world to replace itself, the average woman needs to have around 2.1 children. In the United States, where fertility has been below replacement for about a decade, the average woman now has 1.77.

Several commentators have described the plunge as a mystery, particularly since we’re in a period of economic growth. Some on the right have, absurdly, blamed the shrinking birthrate on abortion, even though abortion rates are also as low as they’ve been since Roe v. Wade was decided. More thoughtful conservatives, like National Review’s David French, speculated that the baby bust could be a sign of the same sort of sweeping despair that has been linked to America’s decreasing life expectancy.

I have another theory. Perhaps the United States is becoming more like the rest of the industrialized world, where declining birthrates are correlated with a lack of support for working mothers.

Outside the United States, the pattern is pretty clear. Developed countries that prioritize gender equality — including Sweden, Norway and France — have higher fertility rates than those that don’t. The world’s lowest fertility rates are in countries that are economically developed but socially conservative, where women have professional opportunities but must shoulder most of the burdens of domestic life. (With its progressive reputation and low birthrate, Germany might seem, on the surface, like an exception, but the country has a tradition of stigmatizing mothers who work outside the home.)