It has long been known that high-income people in the U.S. tend to live longer than those lower down the income ladder. But a growing body of research shows that millions of Americans are, for the first time in more than a century, seeing their life expectancy slip. And, while the reasons for that decline are complex and not yet fully understood, the contours of an explanation are starting to take shape: Despair is shortening their lives.

"One thing we're seeing is that people who are poor live a lot less -- a striking amount less -- than the rich," said Diane Whitemore Schanzenbach, an economist with Northwestern University and director of The Hamilton Project, a think tank focused on boosting the country's economic prosperity. "There are real disparities across the income distribution."

Schanzenbach is co-author of a paper that explores why some Americans are seeing their longevity decline and mortality rates increase. Specifically, over the past three decades the life expectancy of low-income workers has stagnated. or even declined. (Life expectancy is an estimate of how many more years a person has to live at a given age, while the mortality rate refers to the number of deaths over a given period of time.)

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Beyond the simple issues of social justice around why some Americans are dying sooner than others, these trends have enormous implications for everything from how the U.S. seeks to reduce poverty, to Social Security costs, to public investment in health care. More broadly, they demand answers to a key question: Are the eroding financial security, diminishing job opportunities and generally grim economic prospects facing many Americans putting them in an early grave?

Meanwhile, the socioeconomic currents that appear to be driving these changes in life expectancy are also -- as the U.S. presidential election shows -- roiling our politics. At a time when many Americans are rebelling against inequality and questioning the merits of a globalized economy, getting to the bottom of this divergence in lifespans is a matter of national importance.

These findings are not an unrelenting tale of woe, it's worth noting. As Schanzenbach emphasizes, the last century has seen a huge increase in longevity in the U.S. and in other developed nations. Americans as a whole have gained roughly 25 years in life expectancy over that time. Around 1900, the average woman could expect to make it to 48 years, while life expectancy for men was an even shorter 46. Today, those figures stand at 81 for women and 76 for men.

The main factors behind that progress are technological advances, especially in medicine and the production of safer cars. By far the greatest number of deaths in the early 20th century were caused by infections such as pneumonia, influenza, tuberculosis, smallpox and measles.

But the development of antibiotics and vaccines has greatly reduced the toll, leaving cancer and heart disease as the major killers. Improved nutrition, better sanitation and changes in social behavior, such as a decrease in smoking, have also boosted life expectancy.

So it's no surprise that some Americans continue to see improvements in life expectancy. Between 1999 and 2014, the mortality rate for black men aged 45-54 fell by roughly a third, research shows. Infant mortality rates have also declined.

Yet while some Americans can look forward to longer lives and falling death rates, others are seeing the opposite. For the past three decades, the life expectancy of low-income workers has stagnated and even fallen.

As a result, a 40-year-old man in the top 1 percent of income earners in the U.S. (those making at least $389,000 a year) can expect to live to 87. Among the bottom 1 percent, a man of the same age can expect to make it to 72.

Poorer, middle-aged whites, in particular, are being affected. Between 1999 and 2014, mortality rates for this group have risen roughly 10 percent. The main reason, as economists Anne Case and Angus Deaton showed last year: a surge in deaths among people in this group from drug and alcohol abuse and from suicide.

"It's one thing if everybody is having gains, but some are having gains at more rapid paces than others," Schanzenbach said. "But worsening mortality rates are not something we see very often."

Another key finding emerging from the work of economist Raj Chetty of Stanford University is that where you are born and grow up has a profound effect on how long you can expect to live.

He has found that poor Americans tend to have greater life expectancy in cities with larger populations of high-income, better educated people, such as New York and San Francisco. Not surprisingly, perhaps, such cities also tend to spend more on social services. Life expectancy for the poor is lower in metropolitan areas like Detroit and Washington, D.C., where there is less income diversity and poverty is more widespread.

That same gap based on geographic factors is not present for richer Americans.

Exactly what accounts for that -- the relationship between geography and life expectancy -- remains unclear.

Similarly, for now economists and public policy experts are wary of drawing a direct link between the host of problems caused by diminished economic opportunity and rising mortality rates. Other issues, such as the social isolation and depression that often accompanies poverty, almost certainly also play a role. And while the recent surge in suicides and poisoning from drugs and booze reveals a disturbing trend in mortality, it doesn't explain why someone ultimately chooses to end his life.

"We are hard at work trying to connect the dots, but at this point it is just speculative," Case, the Alexander Stewart 1886 professor of economics and public affairs at Princeton University, told CBS MoneyWatch. "One could make a case that as jobs disappear and benefits disappear, it could have a really negative effect on expectations for the future, and that could lead to behavior that cuts people's lives short."

But "It's too early to say how much of this is due to changes in the labor market, GDP or employment practices," she cautioned.

Why have these changes in mortality become apparent only in recent years? One possibility is that the rise in deaths from suicide and poisoning was masked by advances in fighting heart disease. But progress in fighting such diseases has slowed, with the result that deaths that may be linked to what Case calls the "deaths of despair" among certain Americans are pushing up mortality rates.

What is clear, by contrast, is that among the world's richer nations the divergence in life expectancy is a uniquely American problem. Other wealthy countries haven't seen a surge in suicides and deaths cause by substance abuse, while mortality rates for low-income, middle-aged people have continued to decline.

As the research continues, Schanzenbach calls for a shift in how economic issues are thought of and understood. Any discussion of the "economy" is, after all, a discussion about people. That requires going beyond the usual focus on growth figures, inflation and wages to consider the many ways in which the economy is, for better and for worse, a matter of life and death.