(Bloomberg View) -- Stress, I've long suspected, may explain why lifespans have been lengthening for high-income Americans but have remained the same or even shortened for low-income and middle-income people.

A new analysis from the Hamilton Project released today adds important evidence: Biomarkers of stress have risen much more rapidly for low-income people than for high-income ones.

The most recent national data show that life expectancy declined in 2015.

Although these statistics are not broken down by income, it’s a good bet that the greatest declines were concentrated among lower- and middle-income people, because for decades gaps in life expectancy by income have been widening -- as a National Academy of Sciences panel I co-chaired documented.

Our panel was unable to explain why this has been happening, however.

A sharper decline in smoking among high-income people is one partial explanation, but this and other identifiable factors can't account for most of the growing longevity gap.

And so I've come to suspect that the hidden factor is stress: As income inequality has increased, life has become substantially more stressful for low- and moderate-income families than it is for high-income families, and prolonged exposure to such stress is literally killing people.

This has always been just a theory. While there has been plenty of indirect evidence, including the opioid epidemic, there has been little direct empirical support.

One recent study did find an association between workplace stress and a decline in life expectancy, and the link was especially strong for low-income workers. But that study generated its results from several sets of assumptions, and so was only suggestive.

The new Hamilton Project analysis, however, noticeably strengthens the evidence. (I am on Hamilton's advisory council.)

Diane Schanzenbach, Megan Mumford, Ryan Nunn and Lauren Bauer examined changes over time in the relationship between health and income. And, using laboratory measurements of health biomarkers from the National Health and Nutrition Examination Survey, they constructed a new measure of stress load.

That survey is particularly insightful for the purpose at hand because it collects health measurements along with more traditional socioeconomic and related data, and because its data go back several decades.

The Hamilton team looked at the measurements from 1976-80 and 2009-14 to see how things had changed.

Beginning with self-reported health, the researchers found a decline for all income groups, but for high-income people, the change was small and not statistically significant.

For low-income people, the decline was much larger and statistically significant.

The researchers next examined the data on biomarkers that are associated with long-term stress. Note that this approach does not measure the external environment, but rather how one internalizes it.

The measurements involved have all been identified in the medical literature as predictors of illness and mortality: blood pressure, triglycerides and cholesterol (markers of cardiovascular risk); creatinine (kidney function); and albumin (liver function).

The researchers then combined these into a single stress index, based on how much each variable affected self-reported health.

Two features of the results stand out. First, across the board, stress loads have grown heavier since the late 1970s. Second, the increase has been substantially larger for low-income people than for high-income people.

We may never have definitive proof of the stress theory to explain the widening life expectancy gap.

But these new results -- which will come as no surprise to anyone who has read "Hillbilly Elegy," by J.D. Vance -- are strong evidence that stress load plays a crucial role in stagnant or declining life expectancy among America's low- and middle-income families.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

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Stress may be widening the U.S. longevity gap

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