Helen Epstein is the author of “The Invisible Cure: Africa, the West and the Fight Against AIDS.”

THE world’s seven billionth person is about to be born, according to the United Nations Population Division. Before this century ends, there could well be 10 billion of us, a billion more than previously expected. Nearly all of these extra billion people will be born in Africa, where women in some countries bear seven children each on average, and only 1 in 10 uses contraception. With mortality rates from disease falling, the population of some countries could increase eightfold in the next century.

Africa’s future matters to all of us. Globalization, to say nothing of shared interest in scarce resources, connects our fates. In many parts of Africa, people already scramble to obtain food, land and water, and discontent provides fertile ground for extremism. So it is important to think carefully about the response to Africa’s exploding population.

Early next year, researchers will publish findings that provide good, if surprising, news: relaxed, trusting and frank conversations between men and women may be the most effective contraceptive of all.

We know this from Western history. In Europe and the United States, birthrates plummeted between the late 18th and early 20th centuries. A research team led by Ansley J. Coale of Princeton University discovered in the 1980s that this had nothing to do with modern contraceptives, which had not yet been invented, or with government policies. Noting that great shifts in family size seemed to spread like a germ from one Western country to another starting more than two centuries ago, the Australian demographer John Caldwell speculated that the germ was cultural — it was the very idea that childbearing was something couples could discuss openly and decide for themselves.