Chetty and his team found a variety of social and economic factors that seem to be correlated with mobility. As David Leonhardt put it in a New York Times piece summarizing the original study this summer: “All else being equal, upward mobility tended to be higher in metropolitan areas where poor families were more dispersed among mixed-income neighborhoods. Income mobility was also higher in areas with more two-parent households, better elementary schools and high schools, and more civic engagement, including membership in religious and community groups.”

Olinsky and Post found that almost none of those variables were as strongly associated with mobility as the size of a region’s middle class — defined as the percentage of area residents who earn incomes between the 25th and 75th percentiles nationally. That’s related to, but not the same as, income inequality between high and low earners, which the original research team found to be more modestly correlated with mobility.

The original study shows only two variables more strongly associated with mobility than middle-class size, and both were linked to decreased mobility: the region’s share of single mothers and its divorce rate.