Another reason for declining birth rates, Straub and Hadley point out, may be the fact that the conversation on fertility focuses almost totally on women. Any policies designed to tackle it are missing half the picture.

Straub believes we should focus on fertility as a men’s health issue and discuss the benefits of caregiving to fathers. “Only one in 100 men in the EU pause their career to look after a child, for women it’s one in three,” he says. That’s despite mountains of evidence that nurturing a child is good for men’s health.

“We need better data,” says Robin Hadley. Until we record men’s fertility, we can’t fully understand it – or the effect it has on their physical and mental health.

And the invisibility of men in fertility discussions extends beyond records. While there’s more awareness now that young women need to think about their fertility, it’s not a conversation being had among young men.

Men also have a biological clock, says Hadley, pointing to research showing that sperm declines in efficacy after 35. And that is something he thinks more young men need to understand.

So making this invisible group visible is one way of tackling social infertility. And another could be to extend the definition of parenting.

All of the researchers who commented on childlessness were keen to point out that people without children still have a vital part to play in raising them.

It’s called alloparenting by behavioural ecologists, explains Anna Rotkirch. For much of our evolution, a baby had more than a dozen caregivers.

One of the childless men that Dr Hadley spoke to in his research described a family he met regularly at his local football club. For a school project, the two young boys needed a grandparent. But they had none.

He stepped in as their surrogate grandfather for three years and after that, when they saw him at football, they would say, “Hi grandad”. It felt wonderful to be acknowledged in that way, he said.

“I think most childless people actually are involved in this kind of care, it’s just invisible,” says Professor Rotkirch, “That’s not seen in the birth registers, but it’s really important.”