‘I was very excited to spend my life alone, to never get married, to never have kids,’ she recalls. ‘People would be like, “Do you want to hold the baby?” I was one of those who’s like, Singaporean kvinnliga personer “No, you keep it. ”’

They live in the leafy suburbs of Philadelphia, balancing parenthood with full-time jobs as co-chief-executives of a travel company, writing books about pronatalism, and their non-profit projects (to which they donated 44 per cent of their post-tax income last year). They project an image of accentuated preppiness, dressing in ultra-crisp country club, business casual when photographers visit, and are effusive and open to the press. Malcolm starts our interview by saying, ‘Absolutely spectacular to meet you!’

Both dealt with adversity in their own youths. Malcolm, 36, was held by court order in a centre for ‘troubled’ teenagers, where he was told by staff that if he resisted they would simply invent new infractions to keep him locked up. Simone, 35, now needs hormone therapy to menstruate regularly and IVF to conceive a child due to years of anorexia.

Back then, Simone was a textbook anti-natalist. She grew up as the only child of a failed polyamorous ong California hippies, where her understanding of a wedding was ‘everyone puts on masks in the forest and there’s a naked sweat lodge’. She was also a ‘mistake baby’, who watched her mother struggle with shelving her career ambitions.

What changed Simone’s mind was not any kind of Stepfordian conversion but a simple promise from Malcolm that she would not have to surrender her career. So it proved. She stays with each child continuously for their first six months, carrying them in a chest harness while working at a treadmill desk, after which Malcolm handles the bulk of child-raising. She finds she gets a productivity bump with each newborn – ‘You’re up every three hours anyway, so why not knock off some emails?’

These personal epiphanies might not have translated into political ones except for Malcolm’s stint as a venture capitalist in South Korea, where the fertility rate is the lowest in the world at 0.8. He was shocked that nobody seemed to regard this as an emergency.

She took no time off during Octavian’s gestation, answered business calls while in labour, and returned to the office five days after his birth

‘If this was an animal species it would be called endangered,’ says Malcolm. ‘We would be freaking out that they are about to go extinct.’ He begins our interview by speaking without interruption for nearly half an hour, incredibly quickly and with frenetic intensity as if chased by the enormity of what is coming.

Virtually every developed nation is now below replacement rate, and the United Nations predicts that the global average will sink below that line around 2056. By 2100 only seven countries are projected to remain above 2.1, mostly in sub-Saharan Africa, meaning developed nations won’t be able to rely on immigration to keep growing.

The impact on actual population will be delayed by decades and hopefully offset by increasing life expectancy, so our species will probably grow through most of the 21st century before holding steady or starting to shrink (estimates vary).

As she says this, her five-month-old daughter Titan Invictus – the couple refuse to give girls feminine names, citing research suggesting they will be taken less seriously – is strapped to her chest, occasionally burbling, while Malcolm has charge of their two sons Torsten, two, and Octavian, three

Most demographers do not consider this a crisis, according to Bernice Kuang. ‘In pop culture, there’s so much really alarmist talk about fertility and population implosion, and that just doesn’t really come up in the same way in academia,’ she says, noting that we cannot predict the long-term impact of future ‘reprotech’. Many experts also see overall population decline as a good thing, arguing that it will help prevent or mitigate climate change and other problems.