Life Lines 84

MEAN LIFE EXPECTANCY



There are few authenticated life spans that have reached 120 years or more. When we do think of how long we will live, it’s usually a tempered reality that flits about 80 years or so and wishful thinking for most of us when we dream of living to be 100. Our views would be very different in times past. Most people born had a rough time making it past the first year of life. In fact, about half of all children born died in infancy until very recently and even in our own century there was a high infant mortality, similar to olden times, in some parts of the non-industrial world.

When people who prepare vital statistics prepare their annual accounts of births, deaths, marriages, abortions, divorces, and causes of mortality, they have to use a standard that is useful. One such device is a mathematical abstraction called mean life expectancy. You simply get the number of deaths at every age from birth and see how many die in each year and make an average. If, as is true for us today, most babies live to celebrate their first birthday, you’ll end up with 75 to 80 years as our 1990s level of mean life experience. If this were 1900 you’d conclude it is about 50 years. If you went back to Philadelphia in 1776 it would be about 30, a figure not very different from estimates based on gravestones in Roman colonies.

Because mathematical abstractions make sense to mathematicians and rarely to anyone else, the public has a peculiar view that walking in Philadelphia in 1776 would give us crowds of children and teenagers and some robust 20 year olds but relatively few middle aged and old people. That isn’t mean life expectancy, that’s fantasy. Except for the historical differences in clothing, transportation, and buildings, Philadelphia would look as it does today – kids and teens and young adults and lots of middle aged and old people. What you won’t see is the amount of grief most married couples experienced having children. About half their babies died of pneumonia and diarrhea.

There was another myth we believed several years ago and which I still encounter from my students. Certain regions of the world near Tibet (a sort of Shangri-La), in the Caucasian mountains, or in the Andes allegedly have such good health that people who live to 100 are common. According to the myth, they either live on yogurt, are vitalized by high altitude air, or enjoy a vegetarian diet. When I was a visiting professor at Minnesota, I got to meet Zhores Medvedev, a then visiting Soviet dissident and scholar. He studied centenarians in the Caucuses and found that they were no more abundant than in other parts of the USSR. What gave them their long lives was the absence of birth certificates and the awards (including one’s portrait on a postage stamp), producing a mysterious reduction in 90 year olds and an excess of self-proclaimed 100 year olds!

That’s sad news for wishful thinking and our mean life expectancy throughout the twenty-first century is unlikely to exceed 90 years.