For the past thousand years, humans have lived longer and longer than their ancestors. Baring pandemics or wars, the human life expectancy has ticked ever upward. And in recent years, life expectancy has shot even higher, spurred by advances in medicine to combat childhood diseases and now-curable disorders, along with gains in access to health care, education, clean food and drinking water.

But a recent study shows a disruption in that trend, says The New York Times. The study indicates that from 1990 to 2008, some Americans have actually seen their expected lifespan drop. Researchers found that white women who didn’t finish high school lost, as a group, five years off the ends of their lives. White men with the same educational level lost three years. (Other ethnic groups did not see similar declines.)

“The reasons for the decline remain unclear,” says the Times, “but researchers offered possible explanations, including a spike in prescription drug overdoses among young whites, higher rates of smoking among less educated white women, rising obesity, and a steady increase in the number of the least educated Americans who lack health insurance.”

The last time life expectancy dropped on this scale in the United States, the 1918 Spanish influenza pandemic was to blame. The current shift may fall in line with a forecast, made in 2005 by some of the same scientists, that steady increases in obesity may catch up to and hamper the extension of people’s lifespans.

Recent discussions have often focused on an ever-expanding life-expectancy “gap,” where rich people tend to live longer than poor people. These differences have traditionally stemmed from greater gains for the rich than for the poor—the gap grows, but not because one group’s lifespan is diminishing.

On the other hand, this is not the first instance of a true drop in life-expectancy seen in the past few years. Discovery News reported in 2010 that “ife expectancy has dropped slightly for all Americans except for black men, who gained around two and a half months in longevity, according to a recent report.” Those shifts were, however, subtle: drops of just a few tenths of a year, in contrast with the new studies’ suggested drop of a few years.

The recent news could also be an indication that, within the bounds of modern healthcare and technology, humans might actually be starting to run into a wall—we may be hitting peak life expectancy. A 2006 report for Congress says that, overall, “life expectancy at birth for the total population has reached an all-time American high level.” From this high perch, shifts in access to things that further prolong lives can have a powerful effect.

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