- 1: the average length of life of a kind of organism or of a material object especially in a particular environment or under specified circumstances
- 2: the duration of existence of an individual
Life expectancy is the expected (in the statistical sense) number of years of life remaining at a given age. Because life expectancy is an average, a particular person may well die many years before or many years after their "expected" survival. The term "maximum life span" has a quite different meaning. The "median life span" is also a different concept although fairly similar to life expectancy numerically in most developed countries.
The term that is known as life expectancy is most often used in the context of human populations, but is also used in plant or animal ecology; it is calculated by the analysis of life tables (also known as actuarial tables). The term life expectancy may also be used in the context of manufactured objects although the related term shelf life is used for consumer products and the terms "mean time to breakdown" (MTTB) and "mean time before failures" (MTBF) are used in engineering.
Life expectancy increases with age as the individual survives the higher mortality rates associated with childhood. In general, the available data indicate that longer lifespans became more common recently in human evolution. This increased longevity is attributed by some writers to cultural adaptations rather than genetic evolution, although some research indicates that during the Neolithic Revolution natural selection favored increased longevity. Nevertheless, all researchers acknowledge the effect of cultural adaptations upon life expectancy.
During the early 1600s in England, life expectancy was only about 35 years, largely because two-thirds of all children died before the age of four. The average life expectancy in Colonial America was under 25 years in the Virginia colony, and in New England about 40% of children failed to reach adulthood. During the Industrial Revolution, the life expectancy of children increased dramatically. The percentage of children born in London who died before the age of five decreased from 74.5% in 1730–1749 to 31.8% in 1810–1829.
Public health measures are credited with much of the recent increase in life expectancy. During the 20th century, the average lifespan in the United States increased by more than 30 years, of which 25 years can be attributed to advances in public health.
In order to assess the quality of these additional years of life, 'healthy life expectancies' have been calculated for the last 30 years. Since 2001, the World Health Organization publishes statistics called Healthy life expectancy (HALE), defined as the average number of years that a person can expect to live in "full health", excluding the years lived in less than full health due to disease and/or injury. Since 2004, Eurostat publishes annual statistics called Healthy Life Years (HLY) based on reported activity limitations. The United States of America uses similar indicators in the framework of their nationwide health promotion and disease prevention plan "Healthy People 2010". An increasing number of countries are using health expectancy indicators to monitor the health of their population.