Pan and several colleagues argue that the underlying force that drives super-linear productivity in cities is the density with which we're able to form social ties. The larger your city, in other words, the more people you’re likely to come into contact with.
"If you think about productivity, it’s all about ideas, information flows, how easily you can access ideas and opportunities," Pan says. "We believe that the interaction mechanism is what drives the productivity of the city." — theatlanticcities.com