S. JAY OLSHANSKY AND BRUCE A. CARNES
I can imagine no more beneficial change in scientific education than that which would allow [the fields of biology and mathematics] to appreciate something of the imaginative grandeur of the realms of thought explored by the other.
Ronald A. Fisher ( 1958)
For as long as humans have inhabited the earth, intense mortality selection pressures have produced a pattern of high unstable death rates resulting in high attrition at younger ages. Like most other mammals, survival to ages where senescence may be observed has always been a rare event for humans. These highly selective forces of natural selection have come under greater control in the past two hundred years, as we have developed an understanding of the germ theory of disease, and then manipulated our environment to reduce the force of mortality from infectious and parasitic diseases. Recent declines in middle-age and old-age mortality have been observed in nations with low death rates, making survival into older ages a common event in developed nations. Although scientists have been discussing seriously for the past century the evolutionary and biological forces that lead to senescence ( Fisher, 1930; Haldane, 1941; Medawar, 1952; Weismann, 1891; Williams, 1957), these recent unexpected declines in mortality at later ages have brought forth a renewed interest in the question of how high human life expectancy can increase (for example, see Mantonet al., 1991; Olshanskyet al., 1990).
The epidemiological transition of the nineteenth and twentieth centuries has brought forth dramatic demographic changes--including both the rapid growth and ageing of the human population. These changes are already having a profound impact on many aspects of human society, not the least of which include the effects of population growth on the consumption of natural resources and the environment, and the effects of population ageing on age- entitlement programmes and the health status of the population. The distinction between competing estimates of longevity has now become particu-