[[image:Life Expectancy 2007 Estimates CIA World Factbook.PNG|thumb|400px|CIA World Factbook 2007 Estimates for Life Expectancy at birth (years).

Life expectancy is the average number of years of life remaining at a given age. Life expectancy is heavily dependent on the criteria used to select the group. In countries with high infant mortality rates, the life expectancy at birth is highly sensitive to the rate of death in the first few years of life. In these cases, another measure such as life expectancy at age 5 (e₅) can be used to exclude the effects of infant mortality to reveal the effects of causes of death other than early childhood causes.

For the life expectancy of adults rather than from birth see longevity.

See also List of countries by life expectancy

Life expectancy over human history

[[image:Life expectancy 1950-2005.svg|thumb|Life expectancy has been increasing and converging for most of the world. legend: Asia (excluding Middle East) legend: Central America & Caribbean legend: Europe legend: Middle East & North Africa legend: North America legend: Oceania legend: South America legend: Sub-Saharan Africa]] Life expectancy is the average number of years a human has before death, conventionally calculated from the time of birth, but also can be calculated from any specified age.

Advances in sanitation, nutrition, and medical knowledge made possible incredible changes in life expectancy throughout the world, providing subjects for study as well as the need to study them. In the United States, only 50 percent of children born in 1900 were expected to reach the age of 50; life expectancy today is approximately 71 years of age. But note that there is a significant difference between males and female life expectancy - 73.6 years for men and 79.4 years for women. Life expectancy is lower for African Americans; 67.2 years for men and 74.7 years for women (Hoyert, Kochanek, and Murphy, 1999).

Life expectancy increased dramatically in the 20th century. These changes are the result of a combination of factors including nutrition and public health, and medicine only marginally. The most important single factor in the increase is the reduction of death in infancy.

The greatest improvements have been in the richest parts of the world. Life expectancy at birth in the United States in 1900 was 47 years. Life expectancy in India at mid-century was around 32, by 2000 it had risen to 64 years. According to the 2006 WHO Report, due to HIV/AIDS and other health related issues today's life expectancy in poorer nations is almost half that of the industrialized, richer nations 1.

Calculating life expectancy from birth emphasizes contributions to improvement in health at lower ages; low pre-modern life expectancy is influenced by high infant and childhood mortality. If a person did make it to the age of forty they had an average of another twenty years to live. Improvements in sanitation, public health, and nutrition have mainly increased the numbers of people living beyond childhood, with less effect on overall average lifetimes.