Sheridan College graduates prepare for the start of the Sheridan College Commencement Ceremonies at the Bruce Hoffman Golden Dome in Sheridan, Wyo. on Saturday, May 13, 2006.  (National Journal)

When Robert and Helen Merrell Lynd studied Muncie, Ind., during the 1920s for their classic book, Middletown, nothing surprised them more than the lack of working-class resentment toward the business class.

For Middletown's working-class families, most of them dependent on factory jobs, life was often fragile and insecure. Yet, the Lynds found, workers didn't conclude from those conditions that the economic deck was stacked against them. The Lynds "came away puzzled by the fact that the city was becoming methodically divided, but despite that growing disparity there wasn't any overt hostility from the working-class toward that middle [and upper] class," says James Connolly, director of the Center for Middletown Studies at Ball State University in Muncie. "There was a strong sense in the working class that they had a shot [to get ahead], and this went along with a strong tendency to stress the individual's responsibility for their own success or failure."

Throughout the nearly nine decades since, the attitudes the Lynds tracked in Muncie have remained remarkably intact. Pollsters and sociologists have found less antagonism toward the affluent in the U.S. than in most other industrialized nations, precisely because Americans are more likely to believe that anyone with enough skill and determination can reach the top. In that way, faith in the opportunity for upward mobility has defused discontent about income inequality, even as inequality has grown. "Because differences in income in the U.S. are believed to be related to skill and effort, and because social mobility is assumed to be high," Isabel Sawhill, codirector of the Center on Children and Families at the Brookings Institution, wrote recently, "inequality seems to be more acceptable than in Europe."

And yet the gap between those enduring beliefs and our more ambivalent modern realities is widening. The operative definition of the American Dream has long been: In every generation, children will live better than their parents did. Millions of Americans, no matter where they start on the income ladder, still clear that bar. But to a greater extent than our self-image allows, success in America is now a matter of choosing the right parents. As Sawhill and two colleagues have calculated, nearly two-thirds of children born to parents in the bottom fifth of income remain stuck in the lowest two-fifths as adults; by contrast, more than three-fifths of children born into families in the top fifth wind up in the top two-fifths.

A key reason for this trend is that the increasing need for a college diploma to get ahead, ironically, reinforces privilege as much as dissolves it. Why? College is far easier to start than to finish, and children whose parents earned a degree are now five times as likely to graduate as children whose parents did not. The result: Although we consider mobility the heart of the American Dream, international studies find that, in most European countries, children born near the bottom now have a better chance of reaching the top than in the United States.

So far, these new trends haven't shattered the public's belief that success is earned, not inherited. In Allstate/National Journal Heartland Monitor polls over the past four years, a significant majority of Americans said they believe "the American Dream is still possible and achievable for "¦ people like you." Most also think that their own efforts, not events beyond their control, will determine their success. Nothing in those findings would surprise the Lynds.

And yet these polls also point to cracks in that conviction. Nearly three-fifths of Hispanics and African-Americans, and about two-fifths of Asian-Americans, said last year that they believe their children will enjoy greater opportunities. Yet only about a fourth of whites agreed; more than two-fifths of whites feared their children would have fewer opportunities. Some of that may reflect anxiety about racial change. But those attitudes undoubtedly also reflect the limits on mobility that Sawhill and her colleagues have mapped.

As Connolly notes, the faith that hard work brings success "has been one of the elements in our culture that's kept the peace." If that faith falters, the widening disparity in income evident since the 1970s could generate greater resentment than it has so far. A more stratified America may mean a more polarized America. This should worry people already at the top as well as Americans who hope to join them and are starting to doubt if they can.

The writer is the editorial director of National Journal.