Meanwhile, the shift away from rural agriculture and towards urban wage work changed the economics of childbearing. Pre-modern people would not understand the idea that the cost of living is too high to have kids; they would respond that if your costs are too high, you need more kids, since kids help with farm labor. But today, instead of being investment capital, children are a form of consumption.

Contraceptive technology hastened that shift, but academic research does not support the idea that it caused it. Economists attribute at most 40 percent of the post-1957 fertility decline to improved contraceptive access. The first country to undergo a fertility transition was France, in the 18th century, long before modern contraceptives, and academic research has pinpointed numerous cases of discrete economic or cultural shocks that triggered fertility transition quite independently of contraceptive access. Tellingly, the United States first reached birth rates around two children per woman during the Roaring Twenties, not during the Great Depression or after contraception became widely available. Today, fertility rates are falling in countries with rapidly expanding contraceptive access, and also in those without.

Besides those related to contraception, other government policies may matter too, including intergenerational transfers such as Social Security, which reduce fertility by creating a moral hazard. Historically, children were the primary means of old-age support. This support is now state-provided. The state in turn depends on families to produce enough children to finance society-wide retirement programs. An expanding old-age safety net may be a very good thing for many reasons, but one of its adverse consequences has been reduced fertility.

But again, it’s important not to overstate the case here, because in societies with stingy old-age programs, such as those in much of East Asia, fertility rates have fallen even lower.