"They value knowledge over space—that's what the modern city is all about," he said. Successful cities "increase the returns to being smart" by enabling people to learn from one another. In cities with higher average education, even the uneducated earn higher wages; that's evidence of "human capital spillover."
The geography of knowledge exchange is one of intimacy. When I think of the urban experience, the word "intimate" doesn't spring to mind. City living is often lonely, an isolated existence. In fact, a city strength is anonymity. Everyone is from somewhere else. Reinventing yourself is easy.
The big issue with Glaeser's model of urban prosperity concerns density, residential or employment? I can imagine how employment density would catalyze human capital spillover. But residential density? I doubt it matters, at all. Residential density receives most of the attention because city boosters want everyone to love urban living like they do. It's a fetish, confusing correlation and causation. I don't buy the density dividend.
Migration, particularly immigration, is what the modern city is all about. The key word is "cosmopolitan", not "density". Give me suburban county where everyone is from somewhere else (e.g. Fairfax County, VA) and I'll show you an economic powerhouse. Migration is what makes Northern Virginia so special, so dynamic. You can see the world at one strip mall. Different cultures slosh together in unexpected ways.
The very act of moving, particularly to the top tier of global cities, is entrepreneurial. You are surrounded by risk-takers and innovation. The competition is fierce. The cream of the crop is seeking any edge, looking for any opening. Just so happens that this cauldron is usually located in a dense built environment. Migration is what matters.