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Life expectancy is a statistical measure of the average time an organism is expected to live, based on the year of its birth, its current age, and other
demographic factors including sex. The most commonly used measure is
life expectancy at birth (
LEB), which can be defined in two ways.
Cohort LEB is the mean length of life of an actual birth
cohort (all individuals born in a given year) and can be computed only for cohorts born many decades ago so that all their members have died.
Period LEB is the mean length of life of a
hypothetical cohort
[1][2] assumed to be exposed, from birth through death, to the
mortality rates observed at a given year.
[3]
National LEB figures reported by national agencies and international organizations for human populations are indeed estimates of
period LEB. In the
Bronze Age and the
Iron Age, human LEB was 26 years; the 2010 world LEB was 67.2 years. In recent years, LEB in
Eswatini (Swaziland) is about 49, while LEB in
Japan is about 83. The combination of high
infant mortality and deaths in young adulthood from accidents, epidemics, plagues, wars, and childbirth, particularly before modern medicine was widely available, significantly lowers LEB. For example, a society with a LEB of 40 may have few people dying at precisely 40: most will die before 30 or after 55. In populations with high infant mortality rates, LEB is highly sensitive to the rate of death in the first few years of life. Because of this sensitivity to infant mortality, LEB can be subjected to gross misinterpretation, leading one to believe that a population with a low LEB will necessarily have a small proportion of older people.
[4] Another measure, such as life expectancy at age 5 (e5), can be used to exclude the effect of infant mortality to provide a simple measure of overall mortality rates other than in early childhood; in the hypothetical population above, life expectancy at 5 would be another 65.[
clarification needed] Aggregate population measures, such as the proportion of the population in various age groups, should also be used alongside individual-based measures like formal life expectancy when analyzing population structure and dynamics. However, pre-modern societies still had universally higher mortality rates and universally lower life expectancies at every age for both genders, and this example was relatively rare. In societies with life expectancies of 30, for instance, a 40-year remaining timespan at age 5 may not have been uncommon, but a 60-year one was.
Until the middle of the 20th century, infant mortality was approximately 40–60% of the total mortality of the population. If we do not take into account child mortality in total mortality, then the average life expectancy in the 12–19 centuries was approximately 55 years. If a medieval person was able to survive childhood, then he had about a 50% chance of living up to 50–55 years. That is, in reality, people did not die when they lived to be 25–40 years old, but continued to live about twice as long.
[5]
Mathematically, life expectancy is the mean number of years of life remaining at a given age, assuming age-specific mortality rates remain at their most recently measured levels.
[6] It is denoted by
,
[a] which means the mean number of subsequent years of life for someone now aged
, according to a particular
mortality experience.
Longevity, maximum lifespan, and life expectancy are not synonyms. Life expectancy is defined statistically as the mean number of years remaining for an individual or a group of people at a given age. Longevity refers to the characteristics of the relatively long lifespan of some members of a population. Maximum lifespan is the age at death for the longest-lived individual of a species. Moreover, because life expectancy is an average, a particular person may die many years before or many years after the "expected" survival. The term "
maximum lifespan" has a quite different meaning and is more related to longevity.
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