The Census Bureau predicts that China’s population will peak in 2026, just 14 years from now. Its labor force will shrink, and its over-65 population will more than double over the next 20 years, from 115 million to 240 million. It will age very rapidly. Only Japan has aged faster — and Japan had the great advantage of growing rich before it grew old. By 2030, China will have a slightly higher proportion of the population that is elderly than western Europe does today — and western Europe, recall, has a higher median age than Florida.

China, notoriously, has another demographic challenge. The normal sex ratio at birth is about 103 to 105 boys for every 100 girls. In China, as a result of the one-child policy and sex- selective abortion, that ratio has been 120 boys for every 100 girls. From 2000 to 2030, the percentage of men in their late 30s who have never been married is projected to quintuple. Eberstadt doesn’t believe that having an “army of unmarriageable young men” will improve the country’s economy or social cohesion.

He thinks demographic change will pose two problems specific to China. Its society has relied heavily on trust relationships within extended-family networks. In a country where fewer and fewer people will have uncles, those networks will rapidly atrophy. The government, meanwhile, relies for its legitimacy on a level of economic performance that demographic trends imperil.