" Americans who live in metropolitan areas with more than a million residents are, on average, more than 50 percent more productive than Americans who live in smaller metropolitan areas. These relationships are the same even when we take into account the education, experience, and industry of workers. They’re even the same if we take individual workers’ IQs into account. The income gap between urban and rural areas is just as large in other rich countries, and even "

― Edward L. Glaeser , Triumph of the City: How Our Greatest Invention Makes Us Richer, Smarter, Greener, Healthier and Happier