Why the demographic transition happens, though, is obscure—for this reaction by Homo sapiens to abundance looks biologically bonkers. Other species, when their circumstances improve, react by raising their reproductive rate, not curtailing it. And work just published by Anna Goodman of the London School of Hygiene and Tropical Medicine and her colleagues, in the Proceedings of the Royal Society, suggests what humans do is indeed bananas. Dr Goodman has shown that the leading explanation advanced by biologists for the transition does not, in the context of the modern world, actually deliver the goods.
This explanation is that, according to circumstances, people switch between two reproductive strategies. One, known to ecologists as “r-selection”, is to produce lots of offspring but invest little in each of them. This works in environments with high infant mortality. The other, known as “K-selection”, is to have only a few offspring but to nurture them so that they are superb specimens and will thus do well in the competition for resources and mates, and produce more grandchildren for their parents than their less well-nurtured contemporaries. The demographic transition, according to this analysis, is a shift from r-type to K-type behaviour.
My reaction to this is, of course, that biologists may not understand why the demographic transition occurs, but we social scientists do understand it. As far as we know, humans are the only species able to effectively control their own mortality and fertility levels. This happened mostly after Darwin died, of course, so we can't necessarily blame him for the fact that subsequent biologists want to impose the exact same models on humans that they find working in other animals. The article has drawn a lot of on-line comments and my sense is that most readers get it, even if the author of the Economist article (and perhaps of the Royal Proceedings article) did not.