Life expectencies, II (really short ones)

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To follow up on my earlier message on life expectancies, we should talk about the other confusing aspect of them: Why are they sometimes ridiculously low? For example, the present life expectancy for someone living in the USA is 77, which you’d think is perfectly reasonable, but then you hear that the LE of someone living in 1900 was only 40. How could it be possible to nearly double in just 100 years.

The answer is really high infant death rates in 1900 and much lower ones now. Consider my example from before with 100 people, with one dying each year, but now let’s make a little change. In the first year, 25 will die, and thereafter, one each year, until the last dies at age 75. What’s the LE for this group? At the start, just 25, because by that age, 50 of the 100 would have died. Ok, what the LE for those sixty people who survive to age 15? 45 – because thirty will die before age 45, and thirty will die afterwards. Hence, it only took 15 years for the life expectancy to jump 20 years.

In general, any report of a very low LE is being skewed by a very high infant mortality rate. For example, Wikipedia’s Life expectancy page list the LE for “Present day ‘non-civilized’ native groups” as “At birth: 34 At age 15: 54 At age 50: 67” (which oddly, is exactly the same as “Upper Paleolithic” human (ie, cavemen)