How long do countries have until their populations disappear?

AS The Economistreports this week, many women in the richer parts of Asia have gone on “marriage strike”, preferring the single life to the marital yoke. That is one reason why their fertility rates have fallen. And they are not alone. In 83 countries and territories around the world, according to the United Nations, women will not have enough daughters to replace themselves, unless fertility rates rise. In Hong Kong, for example, a cohort of 1,000 women would be expected to give birth to just 547 daughters, at today's fertility rates. (That gives Hong Kong a “net reproduction rate” of just 0.547, in the language of demographers.) If nothing changed, those 547 daughters would be succeeded by just 299 daughters of their own, and so on. At that rate, according to some back-of-the-envelope calculations by The Economist, it would take about 25 generations for Hong Kong's female population to shrink from 3.75m to just one. Given that Hong Kong's average age of childbearing is 31.4 years, it could expect to give birth to its last woman in the year 2798. (That is some time after its neighbour, Macau, which has a higher reproduction rate, but a much smaller population.) By the same unflinching logic, Japan, Germany, Russia, Italy and Spain will not see out the next millennium. Even China, which has a recorded history stretching back at least 3,700 years, has only about 1,500 years left—if present trends continued unbroken.