Presidential candidate and former Pennsylvania Senator Rick Santorum warned this fall that movement "up into the middle income is actually greater, the mobility in Europe, than it is in America," according to The Times. Wisconsin Congressman Paul D. Ryan, a leading House conservative, recently wrote that "mobility from the very bottom up" is "where the United States lags behind."

The story reported that at least five large studies in recent years have found the United States to be less mobile than comparable nations. A Swedish research project found that 42 percent of American men raised in the bottom fifth of incomes stay there as adults. In Denmark, the number was 25 percent. In Britain, it was 30 percent. At the same time, only 8 percent of American men at the bottom rose to the top fifth. That compares with 12 percent of the British and 14 percent of the Danes.

A Canadian study found that just 16 percent of Canadian men raised in the bottom tenth of incomes stayed there as adults, compared with 22 percent of Americans, The Times reported. Similarly, 26 percent of American men raised at the top tenth stayed there, but just 18 percent of Canadians.

Economists argue that a central tool in reviving the middle class - and creating social mobility - is the creation of better-paying middle class jobs. Like so much else, that task is enormously complex. Scholars say the reduction in pay is the product of worldwide economic trends, from technological change to globalization, that are difficult to counter. Harry Holzer, an economist at Georgetown University, tracked which parts of the economy featured high paying jobs over time. The percentage of well-paying jobs provided by the manufacturing sector fell by half - from roughly 27 percent in 1992 to 13.5 percent in 2003.

Holzer notes that the nature of business in the United States changed over the last several decades. In the past, large, capital-intensive manufacturing companies faced relatively little competition from overseas and depended on workers in the United States.

"Big, stable, highly profitable and not very competitive means a bigger pie," Holzer said in an interview. "The simplest thing to do is to cut a bigger slice of the pie for workers."

That business model has disappeared. Globalization caused American firms to face fiercer competition from foreign companies. And technological change allowed American firms to ship manufacturing overseas but still tightly monitor quality. Overall, companies have gained the upper hand on workers, who are increasingly easy to replace.

Paul Osterman, an MIT professor, agreed that those dynamics are irreversible. But he argued that some changes in American business norms unnecessarily accelerated the elimination of middle class jobs. Executives once praised for creating jobs are now rewarded for eliminating them.