IN THIS age of environmental anxiety, it is tempting to believe that the root of the Earth’s troubles is actually very simple: too many people. All our other problems would melt away, if only the population were reduced.

Intuitive – but wrong. When populations go into decline, economies follow. And a healthy economy is required to solve our myriad environmental challenges (see Population: Europe’s problems will grow as it shrinks).

There is a middle way: “demographic sustainability”. This is all about creating both environmental and economic sustainability with a falling, ageing population. It involves fine-tuning fertility to keep populations level or in slow decline. Critically, it also means basing success on stability …