The suburbs didn't quite kill the American Dream, but a particular type did. That's the low-density and racially-polarized suburbs that have defined places like Atlanta. Indeed, as you can see in the chart below from Paul Krugman, there's a noticeable relationship between a metro area's density and its social mobility.

As usual, the elephant in the room here is race. So let's address it: the researchers found that the larger the black population, the lower upward mobility. But on closer inspection, this has something to do with population density too. I went back to the Census data, and looked at the same ten cities Krugman did, but this time I compared their population-density ratios and the percent of their population that's black. There isn't nearly a perfect relationship -- look at Boston or Dallas or Houston -- but there is a relationship.

Now, it's not that suburbs outside the South and Rust Belt are some kind of integrated utopia -- far, far from it -- but rather that density changes things. Well-off whites who work in the city and live close by have an interest in paying for the kind of public goods, like mass transit, that benefit everybody. Well-off whites who live far away don't. Atlanta, of course, is the prototypical case here: going back to the 1970s, it's under-invested in public transit, because car-driving suburbanites haven't wanted to pay for something they think only poor blacks would use (to come, they fear, to their lily-white cul-de-sacs). Even last year, a compromise bill that would have increased the sales tax by 1 percentage points for 10 years to pay for expanded roads and railways in the always-congested city got voted down. This malign neglect of infrastructure keeps low-income people from living near or commuting to better jobs -- and that's not a a race issue. Indeed, the researchers also found that whites and blacks in Atlanta both have a hard time moving up. In other words, racial polarization might spur sprawl, which makes cities less likely to invest in their infrastructure -- and underfunded infrastructure hurts low-income people of all races.

Of course, the story of mass transit isn't just a story about race. There's plenty else going on. Sprawl happens in the Sun Belt, because it can. There's more land. And coastal cities are denser, because they have to be -- though even then, they don't always build better infrastructure. Just look at Los Angeles. But for whatever the reason, upward mobility has a local flavor. And that means part of the solution will too. As Reihan Salam argues, loosening zoning restrictions and building out public transit would let cities become denser and more livable. Both, of course, die a thousand NIMBY deaths in a thousand different cities.

There's an old vision of the American Dream that is obsolete, and has been for quite awhile. That's Thomas Jefferson's idea of a nation of self-sufficient farmers -- an agrarian republic. Over time, as people left the countryside for the cities during the Industrial Revolutions, this vision morphed: it became a nostalgia for (and even snobbery of) small towns. It's a vision that Republicans still cling to. Remember when Sarah Palin talked about "real America"? Or when Republicans warned that high-speed rail and bike lanes were some kind of socialist plot? It's a vision of America at odds with the American Dream today.

It turns out the best place to pursue happiness -- and a career -- is in the city.

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