Education also takes a long time. A woman who studies until she is 25 and then spends ten years building a career has just a few years left to get pregnant before she no longer can. Technology may someday remove this constraint, but for now it is hard to have eight children unless, like Ms Nyero, you start early.
The difference that education makes is especially notable in countries where fertility has only just started to fall. In Ethiopia, for example, a household survey in 2005 found that the fertility rate for women with no formal schooling was 6.1; for women with secondary education or more, it was only 2.0. Educating girls better is one of the few goals that nearly every government agrees is important. So it would be surprising if the girls of the future were worse educated than today’s. The proportion worldwide who complete primary school has risen from 76% in 1997 to 90%. The last mile may be the hardest, but there is no doubt what parents and voters want.
A transition that took 200 years in the West, from seven children to two, can now take place astonishingly fast. When rural folk move to the city, it can happen in a single generation. Consider Dorothy Achieng, a 29-year-old receptionist at an accountancy firm in Nairobi. Her mother had eight children, one of whom died. Dorothy has two. Whereas her mother could barely read and put her older children to work on a small family farm, Ms Achieng hopes to keep hers in school.
Ms Achieng is typical of those who move from the countryside to the city. The rural fertility rate in Kenya is 4.5; the urban one, 3.1. Most of Ms Achieng’s friends, like her, have far fewer children than their parents did. No one she knows has seven or eight children.
Although she lives in a slum and has no running water in her modest two-room flat, Ms Achieng is part of the aspiring middle class. Indeed, on her salary of just $200 a month, she pays for a private school that costs $50 per child, per term. It is a strain, but she thinks it is worth it. She does not plan to have more children. If she did, she says, she could not “give them the best”.
Asked what they want to be when they grow up, her two boys stop whizzing around her flat in pursuit of a remote controlled car. “A doctor,” says Crispian, who is nine. Lennox-Lewis, aged seven, chimes in: “And I want to be a lawyer.”