It is hard to point to one overriding cause, but public health researchers have a few answers. In recent decades, smoking, the single biggest cause of preventable death, has helped drive the disparity, said Andrew Fenelon, a researcher at the Centers for Disease Control and Prevention. As the rich and educated began to drop the habit, its deadly effects fell increasingly on poorer, uneducated people. Jessica Ho, of Duke University, and Mr. Fenelon calculated that smoking accounted for a third to a fifth of the gap in life expectancy between men with college degrees and men with only high school diplomas. For women it was as much as a quarter.

Obesity, which has been sharply rising since the 1980s, is more ambiguous. The gap between obesity rates for high earners and low earners actually narrowed from 1990 to 2010, according to an analysis by the National Academy of Sciences. By 2010, about 37 percent of adults at the lower end of the income ladder were obese, compared with 31 percent at the higher end.

More recently, the prescription drug epidemic has ravaged poor white communities, a problem that experts said would most likely exacerbate the trend of widening disparities.

Limited access to health care accounts for surprisingly few premature deaths in America, researchers have found. So it is an open question whether President Obama’s health care law — which has sharply reduced the number of Americans without health insurance since 2014 — will help ease the disparity.

At the heart of the disparity, said Elizabeth H. Bradley, a professor of public health at Yale, are economic and social inequities, “and those are things that high-tech medicine cannot fix.”

Life expectancy for the bottom 10 percent of male wage earners born in 1920 was 72.9, compared with 73.6 for those born in 1950, the Brookings researchers found. For the top 10 percent, life expectancy jumped to 87.2 from 79.1.