John Magufuli, the president of Tanzania, has strong views about birth control. He does not see the point.

In 2016, he announced that state schools would be free, and, as a result, women could throw away their contraceptives.

On Sept. 9 this year, he told a rally that birth control was a sign of parental laziness. Tanzania must not follow Europe, he went on, where one "side effect" of widespread contraception is a shrinking labor force.

There seems little danger of that. Tanzania's fertility rate is estimated to be 4.9, implying that the average woman will have that many children. Europe's rate is 1.6. Tanzania is helping drive a continental baby boom.

In 1950, sub-Saharan Africa had just 180 million people — a third of Europe's population. By 2050, it will have 2.2 billion — three times as many as Europe. If U.N. forecasts are right, sub-Saharan Africa will have 4 billion people in 2100.

A high dependency ratio

That is worrying, although not for the old reasons. In "An Essay on the Principle of Population," published in 1798, Thomas Malthus claimed that the human population was bound to increase faster than the supply of food, leading to catastrophe.

Although Malthus is still admired by some, the green revolution rubbished his hypothesis. The fear now is not that countries will run out of food but that a surfeit of babies will slow their development.

Magufuli is right to suggest that Europe has many old people and could do with more workers to support them. But Tanzania's many children weigh on its economy, too.

Sub-Saharan Africa's dependency ratio (the population younger than 20 and older than 64 versus the population between those ages) is 129:100, compared with 65:100 in Europe. Sub-Saharan Africa is expected to have a worse dependency ratio than Europe even in 2050.

High fertility can also be seen as a global problem, said Bill Gates, whose foundation (jointly run with his wife, Melinda) will hold a conference this week about the state of the world. Overall, humanity is becoming wealthier. But because birthrates are so high in the poorest parts of the world's poorest countries, poverty and sickness are that much harder to eradicate. "Kids are being born exactly in the places" where it is hardest to get schooling, health and other services to them, he said.

There is nothing inherently African about large families. Botswana's fertility rate is 2.6, down from 6.6 in 1960. South Africa's rate is 2.4. And although the U.N. has a good record of predicting global population growth, it has gotten fertility projections badly wrong in individual countries. Sudden baby busts in countries like Brazil, Iran and Thailand caught almost everyone out. Could Africa also spring a surprise? The U.N.'s demographers project that fertility will fall in every single mainland African country over the next few decades. They just expect a much slower pace of change than Asia or Latin America managed when their families were the same size.

It took Asia 20 years, from 1972 to 1992, to go from a fertility rate above five to below three. Sub-Saharan Africa is expected to complete the same journey in 41 years, ending in 2054. Its fertility rate is not expected to fall below two this century. Because many Africans marry young the generations turn over quickly, leading to fast growth.

Why it might change

The reason the U.N. expects change to be slow in the future is that it has been slow until now.

After stagnating economically in the 1990s, countries like Nigeria and Tanzania grew wealthier in the 2000s. But their fertility rates hardly fell. Nor has urbanization transformed family life as much as you might expect. West Africa is much more urban than east Africa, but has a higher fertility rate.

Three things could drastically change the picture, however. First, more African governments could promote family planning. Ethiopia, Malawi and Rwanda have done so, and their birthrates are dropping faster than average.

The second cause for optimism is education. Broadly, the more girls go to school in a country, the lower that country's birthrate. This seems to be more than just a correlation: several studies, in Africa and elsewhere, have found that education depresses fertility.

To attend school — even a lousy school where you barely learn to read — is to gain a little independence and learn about opportunities that your parents had not envisaged for you.

The third profound change would be stability in the Sahel. The semiarid belt that stretches through Burkina Faso, Chad, Mali, Niger, northern Nigeria and Sudan is lawless in parts and universally poor.

Child death rates are still shockingly high in places. Partly as a result, and also because women's power in the Sahel is undermined by widespread polygamy, people still desire many children. The most recent household survey of Niger, in 2012, found that the average woman thought nine the ideal number.

Progress on all three counts depends mostly on African politicians. It falls to them to create more and better schools, provide security for their people and invest in family planning.

They, not foreign observers, need to conclude that their countries would be wealthier if they had rather fewer children. Like so much in Africa, almost everything depends on the quality of government. And that is hard to decree.