Size of A City's Middle Class Predicts Poor's Economic Mobility

Researchers who worked on a study that some cities seem to create opportunities for upward economic mobility also discovered some interesting correlations. Higher economic mobility was related to the size of the city's middle class. 

"Does the middle class get bigger because economic mobility allows previously poor people to join those ranks? Or do poor people have higher economic mobility because a broad middle class drives opportunity for everyone? A large middle class, for instance, might support a stronger school system, which in turn benefits the lower-income students who attend it. This study can't answer this key causal question."