When the researchers looked for factors that correlate with low or high social mobility, they found, perhaps surprisingly, little direct role for race, one obvious candidate. They did find a significant correlation with the existing level of inequality: “areas with a smaller middle class had lower rates of upward mobility.” This matches what we find in international comparisons, where relatively equal societies like Sweden have much higher mobility than highly unequal America. But they also found a significant negative correlation between residential segregation — different social classes living far apart — and the ability of the poor to rise.

And in Atlanta poor and rich neighborhoods are far apart because, basically, everything is far apart; Atlanta is the Sultan of Sprawl, even more spread out than other major Sun Belt cities. This would make an effective public transportation system nearly impossible to operate even if politicians were willing to pay for it, which they aren’t. As a result, disadvantaged workers often find themselves stranded; there may be jobs available somewhere, but they literally can’t get there.

The apparent inverse relationship between sprawl and social mobility obviously reinforces the case for “smart growth” urban strategies, which try to promote compact centers with access to public transit. But it also bears on a larger debate about what is happening to American society. I know I’m not the only person who read the Times article on the new study and immediately thought, “William Julius Wilson.”

A quarter-century ago Mr. Wilson, a distinguished sociologist, famously argued that the postwar movement of employment out of city centers to the suburbs dealt African-American families, concentrated in those city centers, a heavy blow, removing economic opportunity just as the civil rights movement was finally ending explicit discrimination. And he further argued that social phenomena such as the prevalence of single mothers, often cited as causes of lagging black performance, were actually effects — that is, the family was being undermined by the absence of good jobs.