Hillary Clinton raised the right question, which is a start.

"Why," she asked Monday morning, "do some communities have, frankly, more ladders for opportunity than other communities?"

The likely 2016 Democratic frontrunner was headlining a roundtable discussion at the Center for American Progress on expanding opportunity in urban America. This question is actually a sophisticated and hugely important one, and the fact that Clinton is thinking about it hints at what could be an important theme in the coming election.

By definition, the American Dream sounds like an American phenomenon, something equally accessible to hard workers whether they live in a big city or a rural community, the North or the South, a Rust Belt town or a Sun Belt suburb. But, in fact, an accumulating body of research suggests that children growing up in some parts of the country have much better odds than children elsewhere of climbing up the economic ladder, of rising from poor roots to head middle- and upper-class households of their own.

The American dream, it turns out, is not a universal promise. It's more real for children in Seattle than Atlanta, for poor kids growing up around Salt Lake City than Charlotte.

Clinton cited Monday the research that helped document this, a landmark study led by Raj Chetty and other researchers at Harvard and the University of California at Berkeley released in 2013. They found that a child's prospects for economic mobility vary greatly — and disturbingly — by geography in America. There's something about metropolitan Seattle, in other words, that's more conducive to intergenerational mobility than Atlanta.

So what is that something (or somethings)? A couple of years ago — as recently as the last presidential election — we didn't even know to ask this question. Now that we do, we can have an election-season debate about social mobility that goes far beyond empty platitudes about hard work versus helping hands.

"How do we promote success and upward mobility?" Clinton said on Monday. "It’s not only about average income, as important as that is. You can look at cities that on average have similar affluence, but people are trapped and not able to move up in one city, and are moving up in another."

Metropolitan Seattle and Atlanta have comparable median incomes. But in Seattle, about one in 10 kids raised by families in the bottom fifth of household incomes will rise to the top fifth by age 30. In Atlanta, the same is true for only about one in 25 kids at the bottom.

Chetty, Nathaniel Hendren, Patrick Kline and Emmanuel Saez have offered some initial answers as to what might be going on. Social mobility appears to be higher, they found, in metropolitan areas with less economic and racial segregation, with better schools, more social capital and lower rates of single parenthood. Other researchers at CAP have found higher social mobility among metros with a large middle class.

The importance of good schools isn't surprising. Nor is the role of two-parent families, although part of the finding on this front is fascinating: Even children with married parents have higher mobility when they live in communities with fewer single parents. Perhaps this happens because overwhelmed single mothers are able to contribute less time to not only their own children, but to the communities around them — to the PTA or even the parenting of a neighbor's kids.

The findings about segregation reinforce the idea that social mobility and geographic mobility are intimately linked. If poor communities live segregated far from jobs, as is often the case in a sprawling metro like Atlanta, employment and opportunity are harder to access for poor residents. When poor people are segregated, they're also less likely to benefit from the connections to middle- or upper-income neighbors who might know about a better job opportunity or a good after-school program.

Chetty and his co-authors can't explain all of these relationships; they're just starting the work of highlighting them. But their data raises crucial questions about who we're leaving behind in America and what might be important to help them ahead. And that's precisely the kind of policy debate we might want to have in the upcoming election if we really want to ensure more equality of opportunity.

Clinton's comments Monday suggest that she's already thinking about these problems. Few voters in either party are likely comfortable with the idea that a child's future is significantly determined in the U.S. today by where he or she lives. Talking about the difference between Seattle and Atlanta — as she did Monday — is powerful both because it tugs at the American sense of fairness, and because it turns abstract fears about inequality into something terribly real.

If Clinton talks more about it, the topic gives her a chance to unite many policy goals — investing in better schools, greater job access for the poor, stronger civic institutions like unions and larger middle-class communities — under the much larger theme of social mobility at a time when many Americans worry their children will grow up to be worse off than them.