Moreover, the middle-income group was smaller — and the groups at either extreme larger — in the United States than in any of the 11 Western European countries studied.

And incomes in the middle rose faster in Europe than they did in the United States, according to Pew. Median incomes in the middle tier grew by 9 percent in the United States between 1991 and 2010, compared with a 25 percent gain in Denmark and a 35 percent increase in Britain.

The United States, including the middle class, has a higher median income than nearly all of Europe, even if the Continent is catching up. The median household income in the United States was $52,941 after taxes in 2010, compared with $41,047 in Germany and $41,076 in France.

And while inequality may be widening, the proportion of households in the upper-income strata rose to 15 percent from 13 percent.

“Financially, the U.S. remains well ahead of the countries in Europe,” Mr. Kochhar said. “The difference is how incomes have evolved, and they are catching up.”

Although the cutoff of the study, 2010, may have highlighted weak income gains because it was in the immediate aftermath of the Great Recession, he said that was not enough to alter the study’s findings.

“It’s a clear trend that the middle class in the U.S. is shrinking and not keeping up financially with the upper-income group,” he said. “There is an aura of redistribution of income from middle income to upper income.”