Here's another look at the data on the geography of opportunity in America, this time using the prettier picture the New York Times made. Something a number of people have noticed is that the swath of bad opportunities for poor people seems to largely track the geography of where the African-American population is disproportionately located. That naturally lends itself to the hypothesis that it isn't so much that poor people have bad opportunities in these places as that black people lack upward mobility and happen to be concentrated in the southeast.
But the researchers actually looked at this, and that's not the case. Upward mobility for low-income people of all races is negatively correlated with the size of the local black population.
That could be just a coincidence. But I think it probably isn't. If you look at the more clear-cut case of political opportunity, you'll see that measures such as poll taxes that were meant to disenfranchise black people tended to have the secondary consequence of also disenfranchising poor white people. You could imagine a more generalized version of that. If your poor population contains a very large number of African-Americans, then perhaps the only viable means of keeping the black man down are going to involve denying opportunities for upward mobility to poor people of all races. Strong public schools, economically mixed neighborhoods, dense cities, and other pathways of economic mobility would undermine the racial hierarchy, so they meet with unusual levels of resistance.