Something has gone wrong with the American economy. Over the past 30 years, great technological leaps failed to translate into faster growth, more jobs, or rising incomes. The link between innovation and broad prosperity seems to have broken down.At the heart of the problem is a great migration. Families are fleeing the country's richest cities in droves, leaving places like San Francisco and Boston for the great expanse of the Sunbelt, where homes are cheap, but wages are low.In The Gated City, Ryan Avent, The Economist's economics correspondent, diagnoses a critical misfiring in the American economic machine. America's most innovative cities have become playgrounds for the rich, repelling a cost-conscious middle class and helping to concentrate American wealth in the hands of a few. Until these cities can provide a high quality of life to average households, American economic stagnation will continue.