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For one thing, there are those boats loaded up to the gunwales with black Africans desperate to get across the Mediterranean into Europe. Sub-Saharan Africa is a place an awful lot of people want to escape from. What are the prospects for that situation improving as the population quadruples?

For another, there is the Ebola epidemic, reminding us that Africa is the home of diseases. Before modern medicine, the place was a white man’s graveyard. Mungo Park sailed from England to explore the Niger in January 1805 with 40 companions; by November all but five were dead from disease. Africa’s gift to the 20th century was AIDS. Now ebola is surging. Will this improve when the disease agents have four times as many people to mutate among?

There is also the delicate matter of human capital, of population quality. Why do so many black Africans want out of the place? Bad government and low levels of civilizational attainment. But what accounts for those?

I find smart fraction theory persuasive: sub-Saharan Africa just doesn”€™t have enough clever people. With four times today’s population, black Africa will have four times as many smart people, but unfortunately the same fraction, so the population increase won”€™t help.

We could, I suppose, force the smart people to stay there while draining off the rest as immigrants into our countries. That sounds like just the kind of thing our immigration boosters might go for. Over to you, Messrs. Zuckerberg, Soros, Adelson, Obama …