Excellent piece by Ryan Avent in the NYTimes:
The American economy’s famous upward mobility rested in part on middle-class access to rich, entrepreneurial cities. This machinery is breaking down, however, mostly because upward mobility strikes too many residents of rich places as too messy a pursuit to accommodate. During the Industrial Revolution, for instance, millions of workers flooded into fast-growing cities. This produced slums, but it also allowed poor workers to take advantage of opportunities in new industries, a process that helped create the middle class.
Rapid urban growth would mean denser neighborhoods, which makes many Americans uncomfortable. Preventing this density, however, denies workers access to the best opportunities, constraining the mechanism that helps support a strong middle class.