The entrepreneur Elon Musk and our own Prime Minister may seem unlikely bed-fellows but they have one unassailable demographic fact in common – both have plenty of children. There the similarities end. While Johnson, like his predecessors in government, has yet to adopt pro-natal policies, Musk is strident on the matter of reproduction. We must have more children, he declares, or face civilisational collapse.

Musk’s concerns may seem far-fetched, but civilisation did seem as if it was collapsing a bit when we ran out of tanker drivers and the petrol pumps ran dry earlier this year. It does, too, when we find we can’t get an ambulance for Mum or carer for Dad. And although labour shortages have many causes, demography is at the root of it. With the baby boomers retiring, there are fewer new entrants to the workplace, relative to leavers, than once there were.

This is not a problem which migration can always meet because the UK’s longstanding low fertility is matched across vast and growing swathes of the world. You could walk the length and breadth of Eurasia, from Portugal to Singapore, with every country having a fertility rate below replacement level.

Not very long ago, small family sizes were the sole preserve of wealthy countries, thanks to high levels of education, especially for women, urbanisation and rising incomes. But now even countries which are still quite rural and poor are following suit. Fertility rates have tumbled in unexpected places. In China, they halved from six to three between the late 1960s and late 1970s, before the One Child Policy. The relaxation and now lifting of that policy has made little difference; Chinese fertility rates are below those of the UK. In Iran the rate fell almost as fast in the 1980s and early 1990s. In India, there are now fewer births per woman than required for long-term population stability.

When low fertility first hits, it can offer what is called the “demographic dividend”. Countries full of people in their twenties without multiple offspring to care for can be incredibly buoyant and dynamic. Indonesia, where I have worked, is a case in point. But longer term, trouble hits. It is already affecting places like Bulgaria where decades of low fertility plus emigration mean the population is set to tumble. Germany would be there, too, if it closed its doors to mass migration. Yet this, too, brings its own challenges. If migration is to be from nations with educated people who can fit quickly into the host economy then those people are running out. Poland’s birth rate, for example, is too low now to keep that country going while providing hard-working tradesmen for the rest of Europe.

There is one great exception – sub-Saharan Africa. In some parts of Africa family sizes have decreased. But in West Africa, particularly Nigeria, five or six children per couple remains the norm. A population explosion in one part of the globe while the rest is contracting may prove destabilising. But in many ways we should celebrate the last people on earth who seem interested in perpetuating the human race.