Clusters of smart people of the highly educated sort that economists refer to as "human capital" are the key engine of economic growth and development. Jane Jacobs argued that the clustering of talented and energetic in cities is the fundamental driving force of economic development. In a classic essay, "On the Mechanics of Economic Development," the Nobel prize-winning, University of Chicago economist Robert Lucas formalized Jacobs' insights and argued that human capital, or what can be called Jane Jacobs externalities, are indeed the key factor in economic growth and development. Still, the standard way economists measure human capital is to take the percentage of people in a country, state, or metropolitan area with a bachelor's degree or higher. So I was intrigued by this fascinating analysis by Rob Pitingolo (h/t: Don Peck) which looks at the density of human capital.