Life Expectancy

Defining Life Expectancy

Life expectancy is a statistical projection of the length of an individual's life. Specifically, it is an estimate of [Page 849]the average number of additional years a person can expect to live if the age-specific death rates for a given year prevail for the rest of his or her life. It is a hypothetical measure because it is based on current death rates, yet actual death rates change over the course of a person's life. Consequently, each person's life expectancy changes as he or she ages. Demographers typically calculate two different life expectancy measures: (a) life expectancy at birth, or the number of years a new baby born in a given year can expect to live, and (b) one's life expectancy at age n, or the number of additional years an individual who is n years old can expect to live.

Life expectancy at birth does not simply equal life expectancy at age n plus n years because age-specific life expectancy is selective. That is, individuals who have survived the potentially dangerous years of infancy and childhood are more likely to have an extended life span than is the average member of their birth cohort. For example, life expectancy at birth for a given cohort may be 75 years, yet 75-year-olds in that birth cohort probably can expect to live another 10 years. Life expectancy at birth is lower than life expectancy at 75 years of age because it includes in its calculations those babies who went on to die during infancy, adolescence, or young adulthood. These young ages at death reduce the average life span for members of that birth cohort.

Life expectancies vary widely across nations and within nations by race, ethnicity, and social class. In developing nations, high infant mortality rates contribute to low life expectancies. For instance, life expectancy at birth in Malawi was approximately 46 years in 2006. In contrast, life expectancy at birth in Japan topped 80 years. In addition, life expectancy is affected heavily by “crisis” mortalities that affect large numbers of young persons such as wars and epidemics. For example, life expectancies in many sub-Saharan African nations during the early 21st century range from 35 to 40 years, reflecting high rates of AIDS in nations such as Botswana and Swaziland and reflecting warfare in Sierra Leone and Angola.

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