New research shows that the largest U.S. cities would do well to focus on workers at the bottom of the economic ladder.

Thomas Piketty’s Capital helped make inequality a household word last year by arguing that it's a basic outcome of modern capitalism. “Thomas Piketty won 2014,” announced Slate, and the Upshot points out that inequality has been the theme du jour at the American Economic Association's annual meeting.

Indeed, inequality has become an increasingly dominant feature of U.S. cities. A recent U.S. Conference of Mayors report [PDF] shows that income inequality increased in over two-thirds of U.S. metropolitan areas between 2005 and 2012. The wage gap, meanwhile, nearly doubled from 12 percent to 23 percent in the decade between 2002 and 2012. New York City’s Gini coefficient—the standard measure of income inequality—is now equal to Swaziland’s, Chicago’s almost identical to El Salvador’s, and San Francisco looks like Madagascar.

A pair of new economic studies take on the rising challenge of urban inequality in new and important ways: one by Jung Hyun Choi and Richard K. Green, both of the University of Southern California, and the other [PDF] by Nathaniel Baum-Snow of Brown University, Matthew Freedman of Drexel University, andRonni Pavan of Royal Holloway-University of London.

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The bottom line: inequality is not just an occasional bug of urban economies. It’s a fundamental feature of them, an elemental byproduct of the same basic clustering force that underpins metros’ rise as centers of innovation, startups and economic growth. In other words, the exact same phenomenon of skill clustering that has made tech hubs like San Francisco, New York, and Boston such successes has contributed to the rise of inequality, the growing gap between the haves and the have-nots.