Life expectancy is a summary measure of the average number of additional years a group of people can expect to live at a given exact age. Life expectancy figures are derived from a life table. Life table methodology has been developed for human populations to determine average lengths of life, of healthy life, of married life, and of working life. Indeed, life tables have recently been used to determine the average career length of professional athletes. And life tables have been used to determine the average length of life of nonhumans, including automobiles and animals.
Life expectancy at birth is derived by applying a set of age-specific mortality rates to a hypothetical group of newborns. For example, with data for the year 2000, we could impose the current age-specific mortality patterns of individuals from birth through the oldest ages onto a group of newborns. These calculations are based on mortality rates prevailing today, not in the future; individuals born today may actually experience lower (or possibly higher) mortality one hundred years hence, when they reach age one hundred. Thus, life expectancies represent a current, and not future, measure of survival. Further, period-specific events influence life expectancies. For instance, mortality due to human immunodeficiency virus (HIV), a cause of death that was not evident before the 1980s, affects current life expectancy estimates.
Life expectancy is most commonly used for cohorts of newborns, but can also be reported for other ages, as Table 1 depicts. The first row reveals that individuals born in the United States in 1998 can expect to live an average of 76.7 years, the highest figure ever achieved by individuals in this country. Indeed, in 1900, the average life expectancy at birth was just 47.3 years (Anderson).
The table shows the remaining life expectancy for selected ages. The remaining life expectancy is an additional 72.4 years at age 5 and 3.5 years at age 95. With increasing age, remaining years of expected life generally decreases because individuals have already lived through previous years; but the total life expectancy (age plus remaining years) increases because individuals have already survived earlier ages. Thus, at age 75, the remaining life expectancy is 11.3 years, while the total life expectancy is 86.3 years.
Life expectancy is often confused with life span, a demographic term that refers to the maximum number of years a person can be expected to live under the most ideal circumstances (Nam). Life span for humans is about 120 years. In contrast, life expectancy at birth for individuals in the most long-lived nations around the world is approximately eighty years.
A number of factors influence life expectancies, including socioeconomic status, health behaviors, chronic conditions, sex, race, and ethnicity. Indeed, life expectancy figures are often calculated separately by sex and by race/ethnicity. Life expectancy estimates contribute to aging research by providing an excellent summary measure of the length of life of current and future populations.
RICHARD G. ROGERS ROBERT A. HUMMER PATRICK M. KRUEGER
ANDERSON, R. N. "United States Life Tables, 1997." National Vital Statistics Reports 47 (1999): 1–40.
MURPHY, S. L. "Deaths: Final Data for 1998." National Vital Statistics Reports 48 (2000): 1–106.
NAM, C. B. Understanding Population Change. Itasca, Ill.: FE Peacock Publishers, 1994.
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