Barely Half of 30-Year-Olds Earn More Than Their Parents (ONCE IT WAS NEARLY ALL)

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Reversing the trend will be very difficult, the economists found. If income distribution remains as tilted toward the wealthy as it is now, they calculate, it would take sustained growth of more than 6% a year, adjusted for inflation, to return to an era where nearly all children outearned their parents. Since World War II, the U.S. hasn't experienced anything near that level of growth for a lengthy period of time.

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The paper doesn't provide specific reasons for the declining fortunes of younger Americans, but it generally blames the slowdown in economic growth and, especially, the widening income gap between the wealthy and the rest of society.

"Wages have stagnated in the middle class," said Mr. Chetty in an interview. "When you're in that situation, it becomes very hard for children to do better than their parents."

Mr. Chetty, 37, has explored poverty and income mobility in a series of papers that have gained widespread attention across the political spectrum. His research finds that upward mobility depends heavily on government policies, a position common among Democrats, as well as on neighborhood churches and two-parent families, as Republicans often argue.