How many family did you have

“Very high fertility,” Myrskylä continued, “in particular when mortality is low, creates a rapidly growing population, which requires expansion in the infrastructure and consumes increasingly large amounts of resources.” In Nigeria, the government has attempted to lower its high fertility rate by increasing access to contraceptives and touting the economic advantages of smaller family units.

But families don’t base their desire for children on a society’s optimal number. In many countries in central and West Africa—such as Senegal, Mali, and Cameroon—the desired family size for many young women is four to six children, says John Casterline, a demographer at Ohio State who has conducted research in the region. This number has stayed relatively high even as people have attained higher average levels of education—a shift that, in Asia and Latin America, for instance, is usually accompanied by a shrinking of the hoped-for size of families.

It’s not entirely clear why women’s expectations in these parts of the world haven’t changed as those of women in other regions have. One guess, Casterline says, has to do with how family is conceptualized. “A lot of things in life are perceived as a collective endeavor of a large extended kin group, for the sharing of resources and labor, so that diminishes the personal cost of having a kid,” he told me. “It’s diffused among a larger group of people.” For example, maybe one child is particularly sharp, so his relatives save up to send him to college—“a sort of corporate collective effort,” as Casterline put it—and hope that he gets a high-paying urban job and can help support them.

Another possibility: “There was always the issue of protecting yourself against mortality,” Casterline said, referring to the possibility that a child might not make it into adulthood. He said that child mortality rates in many parts of the world have declined a lot in the past few decades. But they’re still high, and the impulse to hedge against them might linger. “‘How many babies do I need to have now if I’d like to have three adult children in 30 years?’” says Jenny Trinitapoli of the University of Chicago, describing the thought process. “That depends on the mortality rates.”

But these explanations aren’t definitive. Some hard-to-quantify preferences also seem to be playing a role. Casterline remembered conducting surveys in Egypt a decade or so ago, and listening to Egyptians discuss the merits of having three children versus two. “There was some indifference, but there was a real feeling that it’s more of a family—it feels better—to have three children rather than two, because so much of their social life is family gatherings, and having aunts and uncles and cousins,” he says. “And if you have three kids, you get a lot more of that.”

But as the economy and makeup of a society changes, so do people’s preferences, and in that sense, the United States is a telling example. At the beginning of the 19th century, the typical married woman had seven to 10 children, but by the beginning of the 20th, that number had fallen to three. Why? “Children were no longer economic assets who could be put to work,” says Mintz, the historian of childhood.