In 1970, 92 percent of Americans earned more than their parents at age 30. Now barely half do.

Only 51 percent of 30-year-old Americans make more money than their parents did at the same age, economists and sociologists from Stanford, Harvard, and the University of California have learned. The results of their study reflect a shocking decline from four decades prior, when 92 percent of American 30-year-olds in 1970 earned more than their parents did at a similar age.

"My parents thought that one thing about America is that their kids could do better than they were able to do," Raj Chetty, an economist on the research team who emigrated from India at age 9, told The Wall Street Journal. "That was important in my parents' decision to come here."

It isn't immediately clear why Americans aren't earning as much, but economic growth and the widening income gap are likely causes. Regardless, reversing the trend is a daunting task: "If income distribution remains as tilted toward the wealthy as it is now, [the researchers] calculate, it would take sustained growth of more than 6 percent a year, adjusted for inflation, to return to an era where nearly all children outearned their parents," The Wall Street Journal notes. "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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