THE Marie Stopes clinic on the outskirts of Ouagadougou, capital of Burkina Faso, recently performed the country's first vasectomy. It stirred a public outcry. One man said Daoude Zia, a 41-year-old teacher with four children, had let himself be “castrated”. “We must sleep with women without using contraceptives,” wrote another, “and women should throw away family planning because children are a gift of God and we must create more and more.”

African demography is unique. It is the only continent that will double in size, reaching 2 billion people by 2045 at current rates. Some countries, such as Liberia and Niger, are growing faster still, doubling in size in less than 20 years—a stunning increase that is causing forecasts of Malthusian disaster for countries that cannot feed themselves. With 12% of the world's population, sub-Saharan Africa has 57% of the deaths of mothers in childbirth, 49% of its infant mortality and 67% of HIV infections.

Yet Africa is also showing signs of embarking on the same transition towards smaller families that has occurred everywhere else. In north Africa families of two are the norm. Even if you exclude that region, the sub-Saharan part includes areas of relatively low fertility such as southern Africa, where families of three prevail. Big cities, such as Zambia's Lusaka and Congo's Kinshasa, have fertility rates below four; the rate in Ethiopia's Addis Ababa is probably just two. Evidence of lower fertility is raising hopes that Africa can reap a “demographic dividend”, the economic benefit countries get when the share of the working-age population rises relative to children and old people. Which, then, is more likely, dividend or disaster? Start by comparing Africa with other parts of the world. When fertility started to fall in Asia after 1960 and Latin America after 1970, it did so quickly, ineluctably and universally. The number of children a woman could expect in her lifetime fell from six to two in a generation. The fertility fall was continuous. And contraceptive use spread rapidly. Family planners were amazed to discover that only a year or two after contraceptives had appeared in cities, illiterate women were using them in remote villages. The pattern of swift, uninterrupted decline is now taken as the norm: the UN uses it to project a worldwide convergence towards the replacement rate of fertility (2.1, the rate at which a population stabilises in the long term). But convergence is not happening in Africa. In a few countries, including Niger and Uganda, the fall in fertility has barely begun. Where it has started, the decline is usually slower than it was in Asia. East Asian fertility fell by more than half in the 20 years to 1985. In Cameroon fertility has fallen only one point (from 5.7 to 4.7) in the past 20 years. And in eight African countries, including Ghana and Kenya, the decline has stalled—that is, after falling for a while, the rate got stuck at about five. Fertility stalls are not unknown elsewhere: Argentina's fertility remained at three for decades; South Korea and Costa Rica also experienced hiccups. But no continent has experienced so many stalls, or so early in the process of decline, as Africa. In the 1970s the main explanation for the continent's high fertility was cultural. The extended family plays a bigger role in African life than elsewhere; children are often brought up by cousins or aunts. This reduces the burden of child-rearing on the parents and cuts the implicit cost of children. As fertility has begun to fall, though, other explanations have come to the fore.

Family planning is much less readily available in Africa than it was in Asia. By some estimates, a quarter of married women want contraceptives but cannot get them. That reflects reduced aid for family planning in the past 15 years and political ambivalence about cutting fertility in Africa itself. Uganda's president once told a student gathering “your job is to produce children”; a Ugandan village chief says “to avoid having intruders grab our land we must keep producing many children.”

But cultural resistance, lack of contraception or weak political will cannot be the sole explanations. Malawi increased modern contraceptive use from 17% of women in 1998 to 42% in 2010 but fertility fell only a bit, so something else must be going on. To generalise wildly, there are two ways to control fertility: to have children quickly and then use contraception to stop having more, or to space out births, leaving longer intervals between each. Many Africans have traditionally used the second method—and may now be using contraception to make birth intervals even longer. The average lapse between first and second births in South Africa is almost four years. This method of control does cut fertility, but not as much as the other.