The great U.S. economic boom after World War II was an urban phenomenon. Tens of millions of Americans flocked to cities to work and forge a future in the nation’s middle class. And for a few decades, living in the big city paid off. By 1980, four-year college graduates in the most urban quartile of job markets had incomes 40 percent greater, per household, than college graduates in the least urban quartile. And workers without four-year college degrees (“non-college” workers) in the same urban areas had hourly wages 35 percent…