Economists, beginning with Alfred Marshall, have studied the significance of cities in the
production and exploitation of information externalities that, today, we call knowledge
spillovers. This paper presents robust evidence of those effects. We show that patent
intensity—the per capita invention rate—is positively related to the density of employment in
the highly urbanized portion of MAs. All else equal, a city with twice the employment density
(jobs per square mile) of another city will exhibit a patent intensity (patents per capita) that is …