Life Expectancy And Life Span

In the United States in 1900, the average life expectancy (also referred to as longevity) of a newborn baby was 47.7 years—46.4 for males and 49.0 for females. By 1990 the average life expectancy increased to 75.4 years—78.8 for females and 72.0 for males. Why did life expectancy increase so rapidly in the twentieth century, and what are the prospects for increasing it further? Perhaps more important, has the overall health of the population improved or worsened during this transition, and what are the health consequences of further increases in life expectancy?

The measure of life expectancy at birth is a statistic that represents the expected duration of life for babies born during a given time period, usually one calendar year. Calculated from death rates observed at every age, it is based on the critical assumption that the age-specific risks of death observed during a given year will prevail for all babies born in that year, for the remainder of their lives. In contrast, life span is the theoretical upper limit to life that would be observed if everyone in the population adopted ideal lifestyles from birth to death and if external threats to life were eliminated. Some researchers believe that there is no biologically determined life span per se (Carey et al.), but rather a series of time-dependent physiological declines that may eventually be subject to modification.

Life expectancy in the developed nations increased rapidly during the twentieth century because of rapid declines in death rates (usually expressed as the number of deaths per 100,000 population over one year) at younger and middle ages. This transformation in death rates, which has occurred to some extent in every nation, is referred to as the epidemiologic transition (Omran). During this transition, death rates from infectious and parasitic diseases, which tend to kill at younger ages, decline rapidly and the saved population lives to older ages, at which they are exposed to aging-related disorders such as vascular diseases and cancer. Although a small fraction of the population has always survived to older ages, the epidemiologic transition allows over 90 percent of all babies born to survive past the age of sixty-five. The redistribution of death from younger and middle ages to older ages is a general characteristic of the epidemiologic transition, although varying degrees of decline in death rates are experienced by different nations and subgroups of populations within nations.

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