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.

ECONOMIST: Why women live so long

Unlike developing countries, which age from the bottom up because of a falling birth rate, rich countries age from the middle, adding old people with fewer workers to support them, and even fewer children to join the workforce.

ECONOMIST: Economics focus

Since 1995, Dr. Ryff and her Wisconsin team have been studying some 7, 000 individuals and examining factors that influence health and well-being from middle age through old age in a study called MIDUS, or the Mid-Life in the U.S. National Study of Americans, funded by the National Institute on Aging.

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