Population used to be the environmental issue to end all environmental issues — and Paul Ehrlich was responsible. In 1968 Ehrlich, a demographer, and his wife Anne published The Population Bomb, which warned that overpopulation was going to turn the world into a Malthusian hellscape. The book — which was written partially at the suggestion of David Brower, then executive director of the Sierra Club — was influential and popular, selling over 2 million copies. What it wasn’t was right — the Ehrlichs failed to see that fertility rates would fall throughout much of the world, slowing population growth, and that advances in agriculture and technology would allow a much bigger and richer global population to survive and thrive (in most of the world, and at least so far).

Today if someone starts talking about population, chances are worrying that there may be too few of us, not too many — conservatives like Jonathan Last fret that a declining fertility rate in developed nations will drag down the global economy. That’s arguable, especially during a time when our bigger problem seems to be global unemployment, especially among the young. It may also be besides the point — fertility rates have declined largely because people, when given the choice by contraception and changing cultural values, generally don’t want to have lots and lots of kids. Experts can wring their hands — and conservatives like the New York Times columnist Ross Douthat can blame it all on modern decadence — but there doesn’t seem to me much we can do about it. Which is probably for the best — there may be no more personal choice than the decision of if and when to have children.

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At least that’s the case in relatively free societies. As Paul Ehrlich points out in a new Science paper, co-authored with Partha Dasgupta of Cambridge University, in poorer or more repressed societies, social values and institutions can push women to have more children than they might want to. In much of sub-Saharan Africa, where fertility rates are still high, the population bomb is still very real. Understanding why requires a little history lesson. Ehrlich and Dasgupta note that fertility rates dropped significantly in Northwest Europe between the 17th and 18th centuries. One reason: around this time period, it began to become common for couples to set up a new household, separate from their parents, upon marriage. That meant the new couple had to save up resources to prepare to go off on their own, which led to later marriages and fewer children. In England the total fertility rate (TFR) — the average number of children a woman bears over her lifetime — dropped to just four between 1650 and 1700, at a time when, as the authors note, “modern family-planning techniques were unknown and women were mostly illiterate.”

By contrast, nearly every nation in sub-Saharan Africa has TFR levels higher than what England had in 1700 — and not coincidentally, those nations are some of the poorest and most ecologically stressed in the world. There are some unique social factors that Ehrlich and Dasgupta identify that helps explain why family sizes are so large. Fosterage — children living with nonparental kin — is common throughout the region, which reduces the private costs to parents of having children and could in turn encourage them to have more. Communal land tenure, common in the region, is another spur to procreation — more hands are needed to work the land in primarily agricultural societies.

But most importantly, women in these societies too often lack access to contraception — and the education and empowerment needed to use them. Ehrlich and Dasgupta note that very early marriage is common, especially in North Africa and the desperately poor Sahel, which tends to increase the number of children a woman will have over her lifetime. Girls rarely make it to school — just 1 in 100 women in Niger completes secondary school — which again makes it harder for them to have smaller families if they so desire. And many of them do — the proportion of women in Malawi who want to delay their next baby or stop having children but who lack access to contraception is around 25%. As the authors write:

Women who have greater autonomy are better equipped to surmount the many barriers that often prevent easy access to family planning. When the barriers are few, as in Indonesia, the use of contraception and the TFRs among the highest- and lowest-income quintiles are similar (15). When the barriers are numerous, as in the Philippines, the poor both have more children and a greater unmet need for family planning.

Sub-Saharan Africa is so poor that population growth there isn’t likely to have a huge impact on climate change — the average resident of Niger is responsible for 170 times less carbon than the average American. But overpopulation does have a negative impact on the local environment — and just as importantly, it helps keep poor societies poor. There is no shortage of good reasons to support female education and empowerment. Defusing a population bomb is one more.

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