The Malthusian crisis comes not as a result of population growth directly, but because of decreasing specialisation. Increasing self-sufficiency is the very signature of a civilisation under stress, the definition of a falling standard of living.
Until 1800 this was how every economic boom ended: with a partial return to self-sufficiency driven by predation by elites, or diminishing returns from agriculture. It is hard to be sure given the patchy information that this is what happened to Mesopotamia and Egypt after 1500 b c , or India and Rome after a d 500, but it is pretty clear that it happened to China and to Japan in later centuries. As Greg Clark puts it, ‘In the preindustrial world, sporadic technological advance produced people, not wealth.
Having more income means you can afford more babies, but it also means you can afford more luxuries to divert you from constant breeding. Children are consumer goods, but rather time-consuming and demanding ones compared with, say, cars. The transition seems to kick in as countries grow richer, but there is no exact level of income at which it happens, and the poor and the rich within any country start reducing their birth rate about the same time. Once again, there are exceptions: Yemen has almost twice the birth rate and almost twice the income per head of Laos.
Is it female emancipation? Certainly, the correlation between widespread female education and low birth rate is pretty tight, and the high fecundity o f many Arab countries must in part reflect women’s relative lack of control over their own lives. Probably by far the best policy for reducing population is to encourage female education. It is volutionarily plausible that in the human species, females want to have relatively few children and give them high-quality upbringing, whereas males like to have lots o f children and care less about the quality of their upbringing. So the empowerment of women through education gives them the upper hand. But there are exceptions here too: 90 per cent of girls complete primary school in Kenya, which has twice the birth rate of Morocco, where only 72 per cent of girls complete primary school.
Is it urbanisation? Certainly, as people move from farms, where children can help in the fields, to cities where housing is expensive and jobs are outside the home, they find large families to be a drawback. Most cities are – and always have been – places where death rates exceed birth rates. Immigration sustains their numbers. Yet this cannot be the whole story: Nigeria is twice as urbanised and twice as fecund as Bangladesh. In other words, the best that can be said for sure about the demographic transition is that countries lower their birth rates as they grow healthier, wealthier, better educated, more urbanised and more emancipated.
A typical woman probably reasons thus: now I know my children will probably not die of disease, I do not need to have so many; now I can get a job to support those children, I do not want to interrupt my career too often; now I have an education and a pay cheque, I can take control of contraception; now education can get my children non-farming jobs, I shall have only as many as I can support through school; now I can buy consumer goods, I shall be careful not to spread my income across too large a family; now I live in a city I will plan my family. Or some combination of such thoughts. And she will be encouraged by the examples of others, and by family planning clinics.