Starting with 1957, the team looked at actual earnings during the prime working years — the ages of 25 to 55. For a while, it saw a clear pattern: Younger men could expect to make more over their lives than older ones. Every year the starting rewards were higher and kept growing. So men who turned 25 in, say, 1960 would end up with a higher median cumulative income by 55 than men who had turned 25 in 1959. And the ’59ers would, in turn, do better over three decades than those who had turned 25 in 1958.

But that steady progress stopped in the late 1960s. Then, instead of increasing, lifetime earnings for men made an about-face and began to decline. They have been dropping pretty much ever since. The result was that a 25-year-old man who entered the work force in 1967 and worked for the next three decades earned as much as $250,000 more, after taking inflation into account, than a man who had the same type of career but was 15 years younger.

“That’s enough to buy a medium-size house in the United States,” said Fatih Guvenen, an economist at the University of Minnesota and a co-author of the study. “That is what you are missing from one generation to the next generation.”

And the trend appears to be continuing. “Every new cohort made less in median lifetime income than the previous one,” Mr. Guvenen said.

The result is widening lifetime inequality as well. That’s because nearly all of the financial gains have been funneled to those at the top of the income scale. For four out of five men, there was no real growth.

“And it all starts at age 25,” Mr. Guvenen said. The decline in lifetime earnings is largely a result of lower incomes at younger ages rather than at older ages, he said, and “that was very surprising to us.”