Shanna Worthen (aka "Owlfish") has a discussion going on about medieval life expectancy, and she's hit on one of my pet peeves: the misleading way we talk about life expectancy.
My whole life, I've heard life expectancy numbers batted around for the Middle Ages -- and they have always been suspiciously round, usually around 30. As Worthen and her commenters point out, in order to arrive at that sort of figure, you have to do some statistically naive things, and talk about "life expectancy" in a misleading way.
It reminds me of the slick way people use statistics politically. For example, whenever a politician cites a figure that "such-and-such millions of Americans are without private health insurance," he's trying to pull a fast one, because that figure often includes those who are already covered under Medicare and Medicaid. A reasonable person, however, hearing that statistic in a stump speech, is going to assume that those millions of people are without any kind of health care. In fact, usually, the goal of someone using such a figure is to raise the number of people on government health care, thereby ensuring that next year they can say "such-and-such millions more Americans are without private health insurance" -- because those people have left the private market having been picked up by the same government entitlement the pol is pushing. Arguably, what the politician is saying is true, but a reasonable person without the benefit of time to parse the phrase will be misled.
Similarly, the 25 year life expectancy figure is misleading. It suggests that 30 is old, and 50 is absolutely ancient. That's not true, though. People in the Middle Ages lived just as long as people do today -- it's the average that's different. Walk into any village of reasonable size, and you'd find some old gaffer or widow of 80+ years. What you wouldn't find is a Miami retirement community filled with such people.
Why did I use health care as the example of misleading statistics? Because every so often we read in the paper that there's some sort of medical advancement that's going to "raise life expectancy," and the reporter seems to think that means humans will live longer.
We won't. While the average might go up, our bodies still have an expiration date. More and more of us might push 100, but we're not about to start living to 200+. Consider this: If we accept as a given that the average life expectancy of the Middle Ages was 25, then life expectancy has tripled, right? Since we know from both historical and archaeological records that some people lived to 80+ years in the Middle Ages, wouldn't that mean that people are living three times as long? Shouldn't there be some 240 year olds running around, grousing that things just aren't the same since Thomas Jefferson died?
And therein lies the problem. Even if the statistic is accurate, people hear something very different than the statistic is saying. A stat talking about life expectance tripling is about the average tripling, but the way it is popularly perceived is that the length of time people live has tripled. And, of course, it isn't. If you're old enough to read this, a century from now you'll be dead, no matter how much life expectancy rises.