In demography, life expectancy is a statistical measure of the average, or mathematical expected value, of the remaining lifetime of an individual in the given group. For non-human organisms the term lifespan is often used to indicate the average length of life in a given species.
Notice that the 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 10 can be used to exclude the effects of infant mortality to reveal the effects of other causes of death. Usually, though, life expectancy at birth is specified. To calculate it, it is assumed that current mortality levels remain constant throughout the lives of the hypothetical newborns.
Life expectancy over human history
Life expectancy has dramatically increased over the last few centuries of human history. These changes are largely the result of improvements in public health, medicine and nutrition. The greatest improvements have been made in the richest parts of the world, but the same effects are now spreading to other parts of the world as their economies and infrastructure improve.
Life expectancy increased dramatically in the 20th century, especially in developed nations. Life expectancy at birth in the United States in 1901 was 49 years. At the end of the century it was 77 years, an increase of greater than 50%. Similar gains have been enjoyed throughout the world. Life expectancy in India and The People's Republic of China was around 40 years at midcentury. At century's close it had risen to around 63 years. These gains were due largely to the eradication and control of numerous infectious diseases and to advances in agricultural technology (such as chemical fertilizers).
Basic life expectancy numbers tend to exaggerate this growth, however. The low level of pre-modern life expectancy is distorted by the previous extremely 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 medicine, public health and nutrition have therefore mainly increased the numbers of people living beyond childhood, with less effect on overall average lifespan.
These improvements continue to confound the predictions of Thomas Malthus, who predicted what is now known as the Malthusian catastrophe which would occur when population growth exceeded the capacity of the world to sustain that population.
The major exception to this general pattern of improvement has been in those countries worst hit by AIDS, principally in Sub-Saharan Africa, which have seen significant falls in life expectancy due to the disease in recent years.
Another exception is Russia and other former USSR republics after the collapse of the Soviet Union. Life expectancy of men dropped to 59.9 years (below the official retirement age), of women to 72.43 years (1999).
In recent years, obesity-correlated diseases have become a major public health issue in many countries. The prevalence of obesity is thought to have reduced a potential for longer life expectancy by contributing to the rise of cancers, heart disease and diabetes in the developed world.
Throughout human history most of the increase in life expectancy arose from preventing early deaths. However, many scientists believe this will not stay true in the future, as it will be possible to revert aging. According to Leroy Hood, the life expectancy in the next three decades will increase by 10-20 years thanks to advances in DNA sequencing and nanotechnology. Some scientists believe that further advances in medical science may push life expectancy even further, effectively making humans immortal.
Variations in life expectancy in the world today
There are great variations in life expectancy worldwide, mostly caused by differences in public health, medicine and nutrition from country to country.
There are also variations between groups within single countries. For example, in the US in the early 20th century there were very large differences in life expectancy between people of different races, which have since lessened. There remain significant differences in life expectancy between men and women in the US and other developed countries, with women outliving men. These differences by sex have been reducing in recent years, with men's life expectancy improving at a faster rate than women's.
The damaging effects of habits such as tobacco smoking and other addictions also make a significant difference to life expectancy.
Another major influence on life expectancy is occupation. Well-educated professionals working in well-secured office towers have a high life expectancy, while asbestos and coal miners do not. This factor is closely tied to the laws of one's country; political dissidents have short life expectancy in countries where free speech does not exist, and the same is true of gang members in countries where private possession of guns is legal.
- CDC year-by-year life expectancy figures for USA
- Map of life expectancy around the world
- Life expectancy in Roman times
- The changing influence of sex and race on life expectancy in the US