Life expectancyLife expectancy is a statistical measure of the average, or mathematical expected value, of the remaining lifetime of an individual in the given group.
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.
Life expectancy over human history
Life expectancy has dramatically improved 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.
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.
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 - see for example the reference below showing how in the early 20th century race was the principal determinant of life expectancy in the United States, and how that difference has been replaced by sex as the principal determinant over the 20th century.
The damaging effects of habits such as tobacco smoking and other addictions also make a significant difference to life expectancy.