Tuesday, March 03, 2009

Lies about Life Expectancy


One of my pet peeves is umpteen articles which state that in year X or civilization Y, "the average life expectancy was only 45."

The wording of these articles leads one to believe that few people in these periods or cultures lived past 40 or 45.

This is not true. Adult humans have always lived about as long as they do now, ie. 60, 70, 80, 90, 100.

These "average life expectancy" numbers are so low because they include the number of babies who die at or just after birth (infant mortality) and the number of children who die very early in their lives (before 5). Infant and early childhood mortality was much higher in earlier centuries and millennia for all races and ethnicities than today for all types of obvious reasons. Thus, by averaging in a much higher infant and early childhood mortality rate in these periods and cultures, you end up with a much lower 'average life expectancy.'

The fallacy is best illustrated with turtles. Turtles have extremely high mortality rates at birth and in the days and weeks soon after birth: well over 90 percent. But those few turtles who survive these first few weeks have life expectancies of 30, 50, 100, or 150 years. As with humans, a graph of turtle life expectancy is not linear. There is a massive mortality bottleneck in the first months and then life expectancy skyrockets.

Unfortunately, many people read these poorly worded assertions about the average life expectancy of "early man" or 'indigenous, non-white culture X' and interpret these statistics to mean that up until the 1800s, very few people anywhere lived past the age of 45.

That's not true.

Adult humans have always lived about as long as they do today.

The female snapping turtle in the photo above, from Mill Brook in Westbrook, Maine just below Highland Lake, is probably 20-40 years old and will probably live (barring getting run over by a car) for another 20 or 40 or even 60 years. Nobody knows if there is even an upper limit on the age of snapping turtles, except that it is probably like humans, around 120 years.

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