“I was shocked, actually,” Kindig said. “So we went back and did the numbers again, and it came back the same. It’s overwhelming.”

Kindig’s findings were echoed in a July report from University of Washington researcher Chris Murray, which found that inequality in women’s health outcomes steadily increased between 1985 and 2010, with female life expectancy stagnating or declining in 45 percent of U.S. counties. Taken together, the two studies underscore a disturbing trend: While advancements in medicine and technology have prolonged U.S. life expectancy and decreased premature deaths overall, women in parts of the country have been left behind, and in some cases, they are dying younger than they were a generation before. The worst part is no one knows why.

The Kindig study does note strong relationships between county mortality rates and several cultural and socioeconomic indicators. In particular, location appears to have an outsized effect on mortality rates. Counties with rising female mortality rates, marked in red, paint a broad stroke across Appalachia and the Cotton Belt, moving across to the Ozarks and the Great Plains. The Northeast and the Southwest, on the other hand, have been largely untouched.

But it’s not clear how these geographical differences play a role in mortality, or why the effect would be so much greater on women than on men. “Clearly something is going on,” Kindig said. “It could be cultural, political, or environmental, but the truth is we don’t really know the answer.”

Other researchers have pointed out the correlation between education rates and declining female health outcomes. The most shocking study, published in August 2012 by the journal Health Affairs, found that life expectancy for white female high-school dropouts has fallen dramatically over the past 18 years. These women are now expected to die five years earlier than the generation before them—a radical decline that is virtually unheard of in the world of modern medicine. In fact, the only parallel is the spike in Russian male mortality after the fall of the Soviet Union, which has primarily been attributed to rising alcohol consumption and accidental death rates.

“It's unprecedented in American history to see a drop in life expectancy of such magnitude over such a short time period,” said Jay Olshansky, the lead author of the study. “I don't know why it happened so rapidly among this subgroup. Something is different for the lives of poor people today that is worse than it was before.”

Education alone does not explain why female high-school dropouts are so much worse off than they were two decades ago. But researchers have used it as a proxy to determine more significant socioeconomic indicators, like access to health care and income opportunities, as well as health behaviors like smoking and obesity. Smoking in particular appears to have had a significant impact on female mortality rates, as the health consequences of previous decades of tobacco use set in. Olshansky points out that female obesity and drug abuse have risen dramatically over the past two decades, and may also play a role in mortality rates.