Life Expectancy: Don't count on it

Life expectancy (LE) numbers have always confused me. My eyes glazed over at this description for calculating it. The term life expectancy is thrown out all over the place. The way I (naively) looked at it was it was the lower bound on my life if I didn't do anything stupid to get myself killed. So when some one tells me that male LE in the US is 77.5 years, I say to myself, "Great, I'll live at least 77 years!"

Except that is not what it means at all. The LE of a man born in 1900 was 47 years. Yet people born in 1900 that lived to 65 could expect to live 15 more year, or to the age of 80. Umm, so is it 47 or 80? Well, at 65 you've already gotten through a bunch of stuff that kills lesser men (chicken pox, military service, dueling for a woman). So obviously it is a conditional probability.

So if I live to 77, can I then expect to live until 90? Thinking about it in the conditional sheds new insight on how long you will live. If you are pretty sure you can get to 77, then be sure you have a large retirement fund because you could be kicking for a lot longer.

What got me thinking about LE is all the old people in the history books. Many of the people in the history books die relatively old compared to the 47 years figure. If we looked at a distribution of the age of death in 1790 for instance, I think we'd see a large portion in the early forties with a long, thin tail out to about 75 years of age.