Life expectancy is an apparently straightforward term that hides a number of subtleties. From Wikipedia:

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.

Large reductions in infant mortality are responsible for much of the increase in birth life expectancy over the past century. Other contributions come from the reduction in chronic illness and the resulting damage it causes - all damage to a living being is likely to reduce life span according to the Reliability Theory of aging. As James Vaupel put it recently:

Life expectancy [at birth] is a measure of current conditions. It is not a prediction about how long somebody will live. But it's a measure of how long a person, a baby would live if that baby was confined to this year, could not get out of this year, was stuck with the conditions of this year.

It is slowly occurring to me that many folks, mainstream journalists included, use the term "life expectancy" in a fairly imprecise way. It doesn't mean projected life span, and it's not even all that useful to mention life expectancy unless you carefully qualify the group you are talking about. Life expectancy for who, and under what assumptions? There are plenty of examples of this sort of imprecision and mistaken meaning in the current debate over social security.

Posted by Reason

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