As a pro-natalist, I am in full sympathy with the Macron-tweaking mothers, but as a descriptive matter the French president is basically correct. It’s a law of modern Western and East Asian history (we’ll call it Macron’s Law hereafter) that with wealth and education birthrates fall — and fall, and fall. The existence of occasional exceptions only highlights how exceptional they are.

This plunge has happened without population-control interventions as well as with them, and because Western-supported population control efforts in the developing world tended to be inhumane and not-so-mildly racist, over the last couple of decades they have fallen somewhat out of fashion, with Gatesian philanthropists and politicians alike.

So why are they creeping back into the discussion? For three reasons: Because African birthrates haven’t slowed as fast as Western experts once expected, because European demographics are following Macron’s Law toward the grave, and because European leaders are no longer nearly so optimistic about assimilating immigrants as even a few short years ago.

In 2004, the U.N. projected that Africa’s population would level off by 2100 around two billion. Today it projects that it will reach 4.5 billion instead. This change in the expected trend is more likely a result of sluggish economic growth than proof of an African exception to Macron’s Law — though it holds open the possibility that Africa could be such an exception. But whatever the explanation, by century’s end two in five human beings could be African.

This trend would have revived a certain kind of population-bomb anxiety no matter what, but the anxiety in Europe is a little more specific than that — because over the same period, Europe’s population is likely to drop by about one hundred million. (Western Europe’s leaders are a vanguard here: Neither Macron nor Angela Merkel nor Theresa May have any biological children.) In the late 1990s Europe and Africa had about the same population; a hundred years later there could be seven Africans for every European. And the experience of recent refugee crises has demonstrated to European leaders both how easily populations can move northward, and how much harder assimilation may be than they once hoped.