the United States and in many other advanced economies around the world, life expectancy has been on the rise for a very long time. Back in the 1800s, you might have expected to live slightly less than 50 years, but now a woman in the United States can expect to reach the age of 81, and a man is likely to keep going until 76.

One of the most interesting parts of this is the gap between men and women. Last century, there really wasn’t much of a gap, but a difference started to develop as improvements in health care and childbirth mortality vastly reduced the number of women who died prematurely, allowing the natural genetic advantages of being a woman to start to show themselves – thanks in part to our two X chromosomes.

However, that trend seems to have slowed and even reversed in recent decades. A study by the Institute for Health Metrics and Evaluation recently showed that since 1989, women’s life expectancy in the United States only increased by 2.7 years, while men’s life expectancy has risen by 4.6 years. There was some thought that this was simply due to women starting to reach the limits of how far life expectancy could be extended, while men still had some catching up to do, but another 2013 study published in the journal Health Affairs reveals something far more sinister.

What this study found was that for some groups of US women, life expectancy has actually started to fall, and no one knows exactly why. It seems that women in about half of the counties in the US are dying sooner, and ending up at funeral homes such as the American Cremation & Casket Alliance. Many of these are live in rural areas, especially in the South and West.

There are a number of possible reasons for this, including higher smoking rates among the disadvantaged in rural areas, as well as increasing obesity, but there is no consensus among experts on the cause – and this is a question that is being hotly researched as a result.

There is also no clear explanation why this seems to be affecting women rather than men. The researchers looked at premature deaths – deaths that occur before the age of 75 – and found that while the rate of premature deaths among men only rose in 100 counties, the rate for women rose in 1344 counties. The men’s results aren’t statistically significant, but the rates for women were and surprised the researchers.

This is not the first study to look at this type of information, but it is one of the few to look at death rates at the county level. Some other national studies, however, have shown disturbing trends. For instance, one found that white women who did not have a college diploma were also experiencing falling life expectancy, and in general, disadvantaged white women seem to be the most affected by this trend of dying younger.