In 1937, John Maynard Keynes gave a lecture on “Some Economic Consequences of a Declining Population”. Many at the time felt the world was overpopulated and fewer people could only be a good thing, a view that Keynes himself shared. But the purpose of his lecture was to issue a warning: declining populations come with nasty economic side effects.

Keynes, it turned out, was worrying a couple of generations too soon. Births jumped in the postwar baby boom. Today, though, his warning reads like a prophecy. Population is already falling in countries such as Japan and a future global decline is more plausible than ever before. As in the 1930s, many welcome the prospect — largely for environmental reasons — but the economic downside may be more severe than even Keynes anticipated.

Future populations will reflect a truly remarkable fall in global fertility. In rich countries, fertility rates have hovered below replacement levels of 2.1 children per woman for decades, but they are now below that threshold in middle-income countries across the world, from Iran to Thailand to Brazil. In South Korea, the fertility rate dipped to just 0.98 last year, and even in the US it hit an all-time low of 1.73 births per woman.