A new phenomenon, population ageing, is spreading across the Earth. As they modernize, birth rates in more than eighty countries – almost the entire top half of the development tables – have fallen below the replacement minimum of 2.1 births per female per lifetime. Highly correlated with women’s education, sub-replacement fertility rates cut across national cultures, religious heritages, or skin tones, linking Orthodox Russia, Buddhist Thailand, Muslim Iran, Confucian Korea, Catholic Brazil, godless Germany, socialist Sweden, capitalist America... As fewer youths replace their ageing parents, a sustained sub-2.1 birthrate will make a population smaller over time while its median age rises. Japan, the world’s oldest nation, with a median age of 46, is now shrinking by around 200,000 people a year. At current fertility rates, Japan’s median age will be…