Life expectancy
Although it is common usage to talk about life expectancy of any living being (e.g., trees, insects, dogs, stroke victims, mine workers), this article focuses on human life expectancy in general, that is, the aging and longevity profile of the human species.
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Overview
Human life expectancy at various ages and under different circumstances is carefully studied by the insurance and actuarial professions, and is calculated on the basis of historic data as shown on the mortality or annuity table used as a reference.
By way of example, if people that are aged 60 live 10 more years on the average in a country, the life expectancy of people aged 60 in that country is said to be 10. If an age is not specified, life expectancy is understood to be from birth, so that "life expectancy of white Americans are higher than that of Hispanic Americans" is a (correct) statement of fact meaning that white Americans live longer on the average than Hispanics. The fact that an average is calculated over a (sub)set of the population makes life expectancy a statistical measure.
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. European socialist countries (such as the Soviet Union, Poland, Czechoslovakia and Hungary) were characterized by decreasing life expectancy and increasing mortality (especially among adult men) in the late 1960s, 1970s and 1980s. 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.
Life expectancy of Animals
The vast majority of animals have shorter life expectancies than humans do, and typically the lifespan of the animal increases with size. Felines and cetaceans (whales and dolphins) have lifespans in the 20-30 year range, canines and sheep in the 10-20 year range, and large herbivores (cattle, horses, camels, deer) in the 30-50 year range. Birds usually live 10-30 years, with parrots the notable exception. Elephants, parrots, and humans all have lifespans of 50-80 years, and are joint record-holders as far as warm-blooded animals go. Reptiles are highly variable. Turtles are the longest-lived animals; they can sometimes live over 200 years. On the other hand, small mammals such as mice, and invertebrates generally have very short lifespans.Calculating life expectancy
The starting point for calculating life expectancy is to calculate the crude death rates of people in the population at each age. For example, if you observed a group of people who were alive at their 90th birthday, and 10% of them were dead by their 91st birthday, the crude death rate at age 90 would be 10%.
These crude death rates can be used to calculate a life table, from which you can calculate the probability of surviving to each age. In actuarial notation the probability of surviving from age x to age n+x is denoted 
.
The "curtate" life expectancy at age x, denoted 
, is then calculated by adding up these probabilities at every age. This is the expected number of complete years lived (you may think of it as the number of birthdays they celebrate).
On average, it can be expected that people live half a year beyond their final birthday. Because the curtate life expectancy doesn't include the time lived after the final birthday, half a year is added to the curtate life expectancy to calculate the full life expectancy.
Average life expectancy is almost always calculated as an arithmetic mean as above. The median life expectancy is very occasionally used instead.
Note that no allowance has been made in this calculation for expected changes in life expectancy in the future. Usually when life expectancy figures are quoted, they have been calculated like this with no allowance for expected future changes. This means that quoted life expectancy figures are not generally appropriate for calculating how long any given individual of a particular age is expected to live, as they effectively assume that current death rates will be "frozen" and not change in the future. Instead, life expectancy figures can be thought of as a useful statistic to summarise the current health status of a population.
See also
- Age-adjusted life expectancy
- Morbidity
- Mortality
- Demography (Population studies)
- Economics
Increasing lifespan
- John Sperling (see anti-aging movement)
- Longevity
External links
- 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
- Animal lifespans: http://www.tesarta.com/www/resources/library/lifespans.html, http://www.sonic.net/~petdoc/lifespan.htm
