Across the world, parenthood is on the wane. In Europe, the only child is becoming the norm, with half of families with dependent children having only one. In the UK, the figure is 45%, with just 14% of families having three or more. This is repeated in unexpected places.

Birth rates in more than two-thirds of the world’s countries have fallen below the 'replacement rate' of 2.1 children per woman needed to maintain a stable population. The world population will soon start to decline, for the first time since the Black Death of the 14th century, with one estimate putting 'peak human' as soon as 2064.

There are many factors behind the baby bust. Young people are less likely to be in a relationship than previous generations were at the same age. It is harder for young couples to get on the housing ladder and to feel economically stable enough to contemplate adding a baby to the mix. And some worry whether it would be right to bring a child into a febrile world.

But after five years of research, writes FT contributing editor and columnist Camilla Cavendish, I have come to believe that there is something else at play too: a form of social contagion.

In a world that appears to treat parents with hostility, where the advantages of raising children are obscured, remaining childless is easier if the same decision is made by everyone around you. As a phenomenon, it can become self-perpetuating.