It has been a bit of a mystery why women, who normally become infertile in their mid-40s with the arrival of the menopause, often live on and on. Indeed, the average lifespan of women in almost all societies exceeds that of men.

The presumed explanation depends on a more sophisticated understanding of what “fittest” means. It is not enough to survive and have children. Your line has to continue through your grandchildren. In this context, the survival of women beyond menopause makes sense if it translates into extra grandchildren. Proving that hypothesis, though, is difficult. But Mirkka Lahdenpera, of the University of Turku, in Finland, and her colleagues, think they have managed to do so. Their work, just published in Nature, draws on records made in the 18th and 19th centuries in Finland and Canada. The results are striking. In both countries, a woman gained two extra grandchildren for every decade she survived beyond the age of 50.

The post-reproductive survival of humans—women in particular—is truly unusual. Non-reproductive “helpers” of individuals who are breeding are found in many species. But they are usually young animals that have yet to establish themselves, rather than relics from previous generations. The post-reproductive elderly just die. Chimpanzees, for example, have a similar pattern of fertility to people. A female chimp’s fertility peaks in her late 20s, and is more or less extinguished by her mid-40s. But in chimpanzees, mortality rises as fertility declines.

Nor, despite the increase in average lifespan in the rich world over the past few centuries, is post-reproductive survival a modern phenomenon. Until very recently, most of the increase has been due to better survival in childhood and youth, rather than the increased prolongation of middle age into old age. Even in pre-industrial populations, around a third of women were over 45

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