Life expectancy in humans is the average number of years of life remaining for people of a given age, assuming that everyone will experience, for the remainder of their lives, the risk of death based on a current life table. For newborns in the U.S. today, life expectancy is about 77 years.6 Rapid declines in infant, child, maternal and late-life mortality during the 20th century led to an unprecedented 30-year increase in human life expectancy at birth from the 47 years that it was in developed countries in 1900. Repeating this feat during the lifetimes of people alive today is unlikely. Most of the prior advances in life expectancy at birth reflect dramatic declines in mortality risks in childhood and early adult life. Because the young can be saved only once and because these risks are now so close to zero, further improvements, even if they occurred, would have little effect on life expectancy.7,8,9 Future gains in life expectancy will, therefore, require adding decades of life to people who have already survived seven decades or more. Even with precipitous declines in mortality at middle and older ages from those present today, life expectancy at birth is unlikely to exceed 90 years (males and females combined) in the 21st century without scientific advances that permit the modification of the fundamental processes of aging.10 In fact, even eliminating all aging-related causes of death currently written on the death certificates of the elderly will not increase human life expectancy by more than 15 years. To exceed this limit, the underlying processes of aging that increase vulnerability to all the common causes of death will have to be modified.
6Anderson RN. United States life tables, 1998. National Vital Statistics Reports. 2001;48:1-40.
7Olshansky SJ, Carnes BA, Cassel C. In Search of Methuselah: Estimating the upper limits to human longevity. Science. 1990;250:634-640.
8Demetrius L, Ziehe M. The measurement of Darwinian fitness in human populations. Proc R Soc Lond B Biol Sci. 1984;B222:33-50.
9Demongeot J, Demetrius L. La deriv¿ demographique et la selection naturalle: ¿tude empirique de la France (1850-1965). Population. 1989;2:231-248.
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