The report doesn’t measure stress as we typically think about it in daily life. Instead, the researchers track "stress load," an index of certain biological markers such as blood pressure, cholesterol level, and kidney and liver function, that they say are "associated with long-term physiological strain." These metrics are strong indicators of a person's health and mortality, according to the report.

“The poor have seen really striking increases in the stress load index,” said Diane Schanzenbach, one of the report’s authors and the director of the Hamilton Project.

The paper adds to a growing body of research demonstrating that widening inequality in the United States between the rich and the poor is not just an economic phenomenon — it has dramatic effects on health as well.

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The report, which compares surveys carried out by the Centers for Disease Control and Prevention from 1976 to 1980 with surveys from 2009 to 2014, pinpoints several measures of health that have declined over the past decades. While there have been remarkable gains in life expectancy over the nearly 40-year period, the number of Americans who say they are in “very good health” or better has fallen, rates of obesity have risen, and what doctors call “stress load” has spiked.

These metrics have declined for all income groups, but the declines have been smaller for the upper class, which appears to have been insulated by their wealth, the researchers say. In comparison, health metrics for lower- and middle-income people have worsened far more, exacerbating inequality.

In many cases, health metrics for the middle class went from being in the middle of the spectrum to being more in line with those of the poor, as in the obesity rates pictured below. Schanzenbach says she found the pattern surprising. "One thing this points to is that everybody outside of the top is suffering."

The research contains another remarkable finding: that the share of Americans under age 50 reporting excellent or very good health has declined sharply since the 1970s, as the chart below shows. Meanwhile, the share of Americans over age 50 reporting very good health or better has gone up.

Schanzenbach says the researchers are uncertain why the trend is occurring, but it's possible that higher-quality medical care and an increasingly white-collar work experience have improved the relative health of the elderly. Whatever the cause, the trend could be a sign of worsening outcomes in decades to come. “If that’s going to persist, we have a problem,” she says.

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The researchers caution that they still know little about why these effects occur, and that much more research needs to be done.

But they speculate that factors such as job market stress, depression and the quality of health care and diets could be causing the health of lower-income Americans to diverge from their upper-income counterparts. Lower-income Americans who have struggled economically are more likely to experience financial and psychological stress, be exposed to violence and environmental hazards, receive lower quality health care, eat lower-quality food and live in substandard housing, researchers say. Policies like food stamps or housing assistance may help improve the situation, the report says.

Research published earlier this year by some of the same researchers at the Hamilton Project showed that the life expectancy of low-income workers has stagnated or even fallen over the past 30 years. Statistics released last week showed that American life expectancy overall declined in 2015 for the first time in more than two decades, due to a wide range of factors including an opioid epidemic and rising rates of obesity and heart disease.

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One contributing factor may be economic inequality, which research indicates has grown more extreme in the United States in recent decades.

Research published this month from economists at Stanford, Harvard and the University of California at Berkeley suggests that upward mobility has fallen for all Americans in recent decades, but especially for the middle class. The study finds that only half of children born in the 1980s grew up to earn more than their parents, down from 92 percent of children born in 1940.