Opinion

Austad: Life expectancy figures misleading, change rapidly

Every year, as regularly and reliably as the swallows return from the south or lies gush from the lips of politicians, some government agency will release new life expectancy figures that will immediately be misunderstood and misreported by the press. Usually the error goes something like this. “The U.S. Centers for Disease Control reported today that an American child born in 2008 can expect to live 77.8 years.” Although the CDC indeed reported that American life expectancy was 77.8 years in 2008, that number has nothing to do with how long a child born in 2008 can expect to live.

You can't really fault reporters for this error. It arises from the way that scientists sometimes use words in strict technical senses that are different from their common usage. “Life expectancy” sounds like it should refer to the expectation of future life, but it does not. It refers to the past. Life expectancy — shorthand for life expectancy at birth — is the average age at which people in 2008 died. Maybe we should call it death expectancy. Whatever we call it, it is a very precise number that tells us quite a bit about our present state of health. By tracking its change over time, we can see how health has changed historically.

We could calculate life expectancy for children born in 2008, but we would have to wait until all the children born in that year died. Don't hold your breath until that number is available. On the other hand, the Social Security Administration, having a vested interest in these matters, has already forecast (their word) or guessed wildly (my words) that a child born in 2008 can expect to live 82.7 years. Most demographers of future longevity feel this estimate is woefully low. Some, in fact, predict that children born in 2008 can expect to live more than 100 years.

Getting back to things we can actually know, the difference between these two life expectancies — the average of age of everyone who died in a certain year versus the average age at death of everyone born in a certain year — can be substantial because life expectancy, however calculated, is changing at a breathtaking speed. For instance, average age of the Americans who died in 1900 was 47.6 years, whereas the average lifespan of American children born in the same year was more than seven years longer.

Averages such as life expectancy can be misleading, of course. They tend to tell us most when the numbers form something approximating a bell-shaped curve around the average. In that case the average means much the same as the “most common.” Although this is very roughly the case today, for nearly all of human history it was not, because more people died during the first year of life than at any other age. This was true in America, in fact, until 1953. In such cases, the average no longer even approximately represents the most common number. In 1900, when — remember — life expectancy was less than 48 years, the most common age of death for those who survived the first five years of life was 72. Life expectancy in a world of high infant mortality is very misleading in regards to when adults are likely to die.

Life expectancy does tell us that, over the last couple of centuries, we have seen an improvement in human health that is unprecedented. Focusing on the country that has the longest-lived citizens each year (the distinction changes from time to time), life expectancy since 1840 has marched relentlessly upward at the incredible rate of 2.5 years per decade. To put in more graphic terms, that's six hours per day! That, my friends, is progress. What's more, if this trend continues — a topic of heated debate among demographers of longevity — life expectancy, or should I say, death expectancy will reach 100 years by about 2060, long before the children born in 2008 begin thinking about retirement.

Steven Austad is a professor and interim director for the Barshop Institute for Longevity & Aging Studies at the UT Health Science Center San Antonio. His column appears every other Sunday in S.A. Life. Steven.austad@gmail.com