Life expectancy in the U.S. declined slightly in 2016, as it did in 2015, and - at least as important - the overall trends continue to mask increasing disparities across socioeconomic groups.

Carol Graham of the Brookings Institution helps explain why. Her important new book is the empirical version of “Hillbilly Elegy.”

I have long suspected that stress and lack of hope are to blame for widening the gap in life expectancy between lower and higher earners. Graham uses survey data to support this explanation, documenting striking differences in stress and optimism across segments of the population, in her book “Happiness for All? Unequal Hopes and Lives in Pursuit of the American Dream.”

It’s little surprise that low-income Americans report significantly higher levels of daily stress than high-income Americans do. But Graham also notes that the type of stress they typically experience is especially harmful to health, because it seems to be outside the individual’s locus of control. She argues that “stress that is associated with daily struggles and circumstances beyond individuals’ control - as is more common for the poor - has more negative effects than that associated with goal achievement.”

One example of what can cause this type of stress is an unpredictable work schedule. More than 40 percent of early-career hourly workers in the U.S. learn of their work schedules less than a week in advance, recent evidence shows. Among retail and food-service workers, almost 90 percent face variation in at least half of their usual work hours.

Graham also finds major differences across income groups in reported physical pain. Almost 80 percent of people with household income below $24,000 a year reported being in physical pain the day before they were asked, compared with only about 30 percent of those with incomes above $90,000 a year. Her basic findings have been confirmed by David Blanchflower of Dartmouth College and Andrew Oswald of the University of Warwick, using a different data set. These researchers conclude that an astonishing 34 percent of Americans experience bodily aches and pains either often or very often.

The U.S. stands out on many of these measures compared with other countries. The gap in stress levels between low- and high-income people is noticeably smaller in Latin American countries, for example. Low-income American workers are also less likely than Latin Americans to believe that “hard work gets you ahead.” And Blanchflower and Oswald show that reported pain is higher in the U.S. than in any other country they study. “As the U.S. is one of the richest countries in the world, and in principle might be expected to have one of the most comfortable lifestyles in the world,” they note, “it seems strange - to put it at its mildest - that the nation should report such a lot of pain.”

Graham notes one encouraging trend: While the differences among income groups are growing, the gaps between races are shrinking. Life expectancy differences between whites and African Americans are narrowing, even as the gaps by income within each race are widening. And low-income African Americans are quite hopeful about the future - more so even than non-poor white Americans.

After illuminating striking differences across income groups in pain, hope, optimism and stress, Graham is correct in pointing out there are no easy fixes at hand. She’s also right to say that the best way to start to address the gaps is to work to better understand them.

Peter Orszag is a Bloomberg View columnist. He is a vice chairman of investment banking at Lazard. He was President Barack Obama’s director of the Office of Management and Budget from 2009 to 2010 and the director of the Congressional Budget Office from 2007 to 2008.