Their innovation was to measure American income inequality historically. Existing data went back only to the 1970s. Tedious archival research at the Internal Revenue Service allowed them to stretch the data all the way back to 1913.
Once they had collected the data, the computation was easy. They figured out the benchmark for various income levels — the top 10 percent, top 1 percent and top 0.1 percent of earners, for instance — and calculated what share of income each group took each year.
What they found startled them. As in other industrially advanced countries, income inequality in the United States fell after World War II, a period that economic historians call the “Great Compression,” and remained stable through much of the 1970s.
But then inequality started increasing again, with the top 1 percent of earners drawing a bigger and bigger share of overall income. Their graph showing the trend became well-known: a deep U, with inequality as acute today as it was just before the depression.
When they first published their work, income inequality was mostly off the political radar screen, thanks to the 1990s boom, Mr. Saez said.
“Growing inequality was not perceived to be an issue because the economy was growing fast and even the incomes of the 99 percent were growing significantly,” he said.
But the deep downturn of the last few years, and Mr. Obama’s election, brought the issue back to the fore. Peter R. Orszag, the former Obama budget director, has said the Piketty-Saez work “helped to point the way for the administration in its pledge to rebalance the tax code.”
Now living many time zones apart, Mr. Piketty and Mr. Saez update their work with frequent e-mails, Skype conversations and data-sharing through Dropbox.
They have found that the trends have mostly continued. From 2000 to 2007, incomes for the bottom 90 percent of earners rose only about 4 percent, once adjusted for inflation. For the top 0.1 percent, incomes climbed about 94 percent.
The recession interrupted the trend, with the sharp decline in stock prices hitting the pocketbooks of the rich. But the income share of 1 percent has since rebounded. Data that the two economists released in March showed that the top 1 percent of earners got nearly every dollar of the income gains eked out in the first full year of the recovery. In 2010, the top 10 percent of earners took about half of overall income.
That has led the two economists to renew their calls for higher rates on the rich. Along with Peter Diamond, an emeritus professor at M.I.T. and a Nobel laureate, Mr. Saez has estimated the “optimal” top tax rates for the wealthy to be between 45 and 70 percent.
“The debate in Washington is between the Bush-era and Clinton-era tax rates,” said Mr. Diamond, whom Mr. Obama nominated to the Federal Reserve and Republicans blocked. “Our finding is that the debate should be between the pre-1986 Reagan tax rate, which was 50 percent, and the rates that existed from Johnson until Reagan,” which were higher.
“Thirty percent is three times smaller than the 91 percent of Roosevelt,” Mr. Piketty said, responding to the Buffett Rule proposal and referring to the presidency of Franklin D. Roosevelt, who engineered the New Deal. “And inequality is greater than in the time of Roosevelt.”