In the long run, quipped John Maynard Keynes, we’re all dead. That partly explains why in the very long term, population growth has little impact on output per head or returns on capital. More people put strain on resources, but also have more ideas about how to use them effectively.

On a shorter time-scale of mere decades, demographics can have a dramatic impact. Take China, where birth rates have fallen far more swiftly since 1970 than pretty much anywhere else in recorded history. Together with rising life expectancy, that has meant many more workers, with relatively few children and elderly. Those shifts probably account for about 15 per cent of its economic growth between 1982 and 2000.