What explains the differences? The study identified a few potential factors, which my colleague Matt O'Brien has written about. But one was the size of the middle class. Using Chetty's & Co.'s data, CAP's researchers have teased out that relationship a bit more, producing the graph below. As it shows, metro areas with a bigger proportion of middle class families tend to have more mobility.

Chetty's study looked at the relationship between mobility and 28 different factors. According to CAP's math, the size of the middle class had a stronger relationship to it than 26 of them. It even appeared to be more important than the concentration of poverty in a metro area.

Now, the first big caveat: CAP is measuring correlation here, and as always correlation does not equal causation. That said, the researchers make a fairly intuitive and reasonable case for why the presence of middle class families in a city might also make it more economically mobile. Middle class communities tend to create middle class institutions, like well-funded public schools. And it doesn't seem coincidental that Chetty's study found that strong elementary schools were also tightly connected to upward mobility.

Now, the second caveat: the Washington Post's Jim Tankersley spoke with Chetty, who cautioned that, for the 50 largest metro areas —which is to say, where more than half the country lives—"his group's research shows the middle-class/mobility relationship breaks down." The more important issue, Chetty says, is whether or not communities segregate along class lines. The children of low-income families fare better when they grow up near middle class or wealthy households than if they're marooned in a totally impoverished part of town.

That said, if the existence of a healthy middle-class really in any way, or in any circumstances, a pre-requisite to moving families out of poverty over time, it seems like a reason for concern. The more of the middle class that gets chipped away, the harder it will be to build back up.