Evolutionary theory predicts that if you are a mammal growing up in a harsh, unpredictable environment where you are susceptible to disease and might die young, then you should follow a "fast" reproductive strategy - grow up quickly, and have offspring early and close together so you can ensure leaving some viable progeny before you become ill or die. For a range of animal species there is evidence that this does happen. Now research suggests that humans are no exception.

Certainly the theory holds up in comparisons between people in rich and poor countries. Bobbi Low and her colleagues at the University of Michigan at Ann Arbor compared information from nations across the world to see if the age at which women have children changes according to their life expectancy (Cross-Cultural Research, vol 42, p 201). "We found that the human data fit the general mammalian pattern," says Low. "The shorter life expectancy was, the earlier women had their first child."

But can the same biological principles explain the difference in behaviour between rich and poor within a developed, post-industrialised country? Nettle, for one, believes it can. In a study of over 8000 families, he found that in the most deprived parts of England people can barely expect 50 years of healthy life, nearly two decades less than in affluent areas. And sure enough, women from poor neighbourhoods are likely to have their babies at an early age and in quick succession. They have smaller babies and they breastfeed less, both of which make it easier to get pregnant again sooner (Behavioral Ecology, DOI: 10.1093/beheco/arp202).

Well, when you put it like that...