Emmanuel Saez and Thomas Piketty have spent the last decade tracking the incomes of the poor, the middle class and the rich in countries across the world. More than anything else, their work shows that the top earners in the United States have taken a bigger and bigger share of overall income over the last three decades, with inequality nearly as acute as it was before the Great Depression....

“In a way, the United States is becoming like Old Europe, which is very strange in historical perspective,” Mr. Piketty said. “The United States used to be very egalitarian, not just in spirit but in actuality. Inequality of wealth and income used to be much larger in France. And very high taxes on the very rich — that was invented in the United States,” he said.

Mr. Saez added, “Absent drastic policy changes, I doubt that income inequality will decline on its own.”

The two economists’ project of mapping income inequality started two decades ago, when Mr. Saez was teaching at Harvard and Mr. Piketty teaching down the road at the Massachusetts Institute of Technology.

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.