You can see what my colleague Derek Thompson calls the geography of the American Dream in the map below. It shows where kids have the best and worst chances of moving up from the bottom to the top quintile—and that the South looks more like a banana republic. (Note: darker colors mean there is less mobility, and lighter colors mean that there's more).

So what makes northern California different from North Carolina? Well, we don't know for sure, but we do know what doesn't. The researchers found that local tax and spending decisions explain some, but not too much, of this regional mobility gap. Neither does local school quality, at least judged by class size. Local area colleges and tuition were also non-factors. And so were local labor markets, including their share of manufacturing jobs and those facing cheap, foreign competition. But here's what we know does matter. Just how much isn't clear.

1. Race. The researchers found that the larger the black population, the lower the upward mobility. But this isn't actually a black-white issue. It's a rich-poor one. Low-income whites who live in areas with more black people also have a harder time moving up the income ladder. In other words, it's something about the places that black people live that hurts mobility.

2. Segregation. Something like the poor being isolated—isolated from good jobs and good schools. See, the more black people a place has, the more divided it tends to be along racial and economic lines. The more divided it is, the more sprawl there is. And the more sprawl there is, the less higher-income people are willing to invest in things like public transit.

That leaves the poor in the ghetto, with no way out for their American Dreams. They're stuck with bad schools, bad jobs, and bad commutes if they do manage to find better work. So it should be no surprise that the researchers found that racial segregation, income segregation, and sprawl are all strongly negatively correlated with upward mobility. But what might surprise is that it doesn't matter whether the rich cut themselves off from everybody else. What matters is whether the middle class cut themselves off from the poor.

3. Social Capital. Living around the middle class doesn't just bring better jobs and schools (which help, but probably aren't enough). It brings better institutions too. Things like religious groups, civic groups, and any other kind of group that keeps people from bowling alone. All of these are strongly correlated with more mobility—which is why Utah, with its vast Mormon safety net and services, is one of the best places to be born poor.

4. Inequality. The 1 percent are different from you and me—they have so much more money that they live in a different world. It's a world of $40,000 a year preschool, "nanny consultants," and an endless supply of private tutors. It keeps the children of the super-rich from falling too far, but it doesn't keep the poor from rising (at least into the top quintile). There just wasn't any correlation between the rise and rise of the 1 percent and upward mobility. In other words, it doesn't hurt your chances of making it into the top 80 to 99 percent if the super-rich get even richer.