Monday, February 20, 2012

How Long Do We Live?

Common wisdom tells us that in our era, people tend to live longer lives; a century or two ago, life spans were shorter, and during the Middle Ages, they were very short. Right? Maybe not. Our lives may not actually be that much longer than those of our ancestors.

How did the general impression arise that we now have much longer life spans? One of the chief culprits is ignoring the difference between 'average life span' and 'life expectancy'. These two phrases sound similar enough that we care inclined to think that, because the average life span in the Middle Ages was shorter than current life expectancy, we are living much longer nowadays. A Washington Post article tells us that:

To hear that the average U.S. life expectancy was 47 years in 1900 and 78 years as of 2007, you might conclude that there weren’t a lot of old people in the old days — and that modern medicine invented old age. But average life expectancy is heavily skewed by childhood deaths, and infant mortality rates were high back then. In 1900, the U.S. infant mortality rate was approximately 100 infant deaths per 1,000 live births. In 2000, the rate was 6.89 infant deaths per 1,000 live births.

To be sure, we are living longer. But the difference may not be as much as we think, given the nature of the statistics. 'Average life span' is what it seems to be: on average, how long a person lives. But the phrase 'life expectancy' is used to factor out infant mortality, and unusual statistical hiccups reflecting large numbers of unexpected deaths: 'life expectancy' is how long a person will probably live, if she or he has made it through childhood, and if nothing drastically unexpected happens (a war, a famine, a plague, or a hurricane); it's an attempt to capture a person's natural life span. Declines in infant mortality have boosted average life spans, but don't really change life expectancies:

The bulk of that decline came in the first half of the century, from simple public health measures such as improved sanitation and nutrition, not open heart surgery, MRIs or sophisticated medicines. Similarly, better obstetrical education and safer deliveries in that same period also led to steep declines in maternal mortality, so that by 1950, average life expectancy had catapulted to 68 years.

Imagine the statistical skewing which would result from factoring out death deaths caused by WWI and WWII. Those would be huge numbers. So average life spans in the Middle Ages might seem short because they include the millions of deaths resulting from the Thirty Years' War and the Black Death plague, but the life expectancy of twentieth-century populations factors out the war casualties.

Statistics about life in the Middle Ages are to some extent guesswork; we can make much more precise comparisons about recent decades. Were lives really that much shorter a hundred years ago? Maybe not:

For all its technological sophistication and hefty price tag, modern medicine may be doing more to complicate the end of life than to prolong or improve it. If a person living in 1900 managed to survive childhood and childbearing, she had a good chance of growing old. According to the Centers for Disease Control and Prevention, a person who made it to 65 in 1900 could expect to live an average of 12 more years; if she made it to 85, she could expect to go another four years. In 2007, a 65-year-old American could expect to live, on average, another 19 years; if he made it to 85, he could expect to go another six years.

The lesson? Life in our era is, in some ways, very different than life in previous eras; but in other ways, our existence isn't that different. The task is to determine what the similarities are, and what the differences. One of the similarities is life span: despite our mental images of the previous eras, lives weren't that much shorter in the past.