In more and more countries, women started having fewer children than the number required to keep populations stable. Four out of nine people already live in countries in which the fertility rate has dipped below the replacement rate. Last year the United Nations said it thought the world's average fertility would fall below replacement by 2025.

If the world's population does not look like rising or shrinking to unmanageable levels, surely governments can watch its progress with equanimity? Not quite.

In Japan

that workforce will shrink by a fifth in the next decade—a considerable loss of knowledge and skills. At the other end of the age spectrum, state pensions systems face difficulties now, when there are four people of working age to each retired person. By 2030, Japan and Italy will have only two per retiree; by 2050, the ratio will be three to two.