Life expectancy is a statistical estimate of how long an organism is predicted to live based on its birth year, present age, and other demographic characteristics such as gender. The most often used
metric is life expectancy at birth (LEB), which has two definitions. Cohort LEB is the average length of life of a birth cohort (all persons born in a particular year) and can only be calculated for
cohorts born so long ago that all members have died. Period LEB is the average length of life of a hypothetical cohort supposed to be exposed to the mortality rates reported in a particular year from
birth to death.
National LEB data for human populations given by national agencies and international organizations are period LEB estimates. Human LEB was 26 years in the Bronze and Iron Ages; the globe
LEB in 2010 was 67.2 years. In recent years, the LEB in Eswatini (Swaziland) has been 49, whereas the LEB in Japan has been 83. The combination of high infant mortality and fatalities in early
adulthood from accidents, epidemics, plagues, wars, and childbirth prior to the widespread availability of modern treatment dramatically reduces LEB.
For example, in a society with an LEB of 40, few people will die at exactly 40: most will die before 30 or beyond 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, LEB can be greatly misconstrued, leading to the mistaken notion that a community with a low LEB has a low proportion of
elderly persons.
To offer a straightforward estimate of total death rates other than in early childhood, a separate measure, such as life expectancy at age 5 (e5), can be employed to eliminate the influence of infant
mortality. When examining population structure and dynamics, aggregate population metrics such as the proportion of the population in various age groups are utilized with individual-based
indicators such as formal life expectancy. However, pre-modern cultures had uniformly greater death rates