Detroit is a symbol of the old economy’s decline. It’s not just the derelict center; the metropolitan area as a whole lost population between 2000 and 2010, the worst performance among major cities. Atlanta, by contrast, epitomizes the rise of the Sun Belt; it gained more than a million people over the same period, roughly matching the performance of Dallas and Houston without the extra boost from oil.

Yet in one important respect booming Atlanta looks just like Detroit gone bust: both are places where the American dream seems to be dying, where the children of the poor have great difficulty climbing the economic ladder. In fact, upward social mobility — the extent to which children manage to achieve a higher socioeconomic status than their parents — is even lower in Atlanta than it is in Detroit. And it’s far lower in both cities than it is in, say, Boston or San Francisco, even though these cities have much slower growth than Atlanta.

So what’s the matter with Atlanta? A new study suggests that the city may just be too spread out, so that job opportunities are literally out of reach for people stranded in the wrong neighborhoods. Sprawl may be killing Horatio Alger.

The new study comes from the Equality of Opportunity Project, which is led by economists at Harvard and Berkeley. There have been many comparisons of social mobility across countries; all such studies find that these days America, which still thinks of itself as the land of opportunity, actually has more of an inherited class system than other advanced nations. The new project asks how social mobility varies across U.S. cities, and finds that it varies a lot. In San Francisco a child born into the bottom fifth of the income distribution has an 11 percent chance of making it into the top fifth, but in Atlanta the corresponding number is only 4 percent.

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.

These days, you hear less than you used to about alleged African-American social dysfunction, because traditional families have become much weaker among working-class whites, too. Why? Well, rising inequality and the general hollowing out of the job market are probably the main culprits. But the new research on social mobility suggests that sprawl — not just the movement of jobs out of the city, but their movement out of reach of many less-affluent residents of the suburbs, too — is also playing a role.

As I said, this observation clearly reinforces the case for policies that help families function without multiple cars. But you should also see it in the larger context of a nation that has lost its way, that preaches equality of opportunity while offering less and less opportunity to those who need it most.