There is no definitive cause, and explanations vary. Mathew Kahn, an economist at UCLA, e-mailed, “Educated liberals are tolerant people who are willing to live in racially integrated areas even if the minority neighbors are poor. Such liberals are more willing to vote for redistributionist policies and this may attract poor people to collect such transfers.”

For an extreme example of rich and poor living cheek-by-jowl, consider New York’s 14th congressional district, on Manhattan’s Upper East Side. One of the very wealthiest of the nation’s 435 congressional districts, it is located 2 miles from New York’s 16th congressional district in the Bronx, which is one of the nation’s very poorest. As any New Yorker can tell you, however, the cheeks don’t necessarily live on the same blocks as the jowls. That 2 miles could just as easily be 200.

Housing prices influence inequality. In the Journal of Urban Economics, Kahn cited resistance to growth and sprawl in liberal California cities such as Santa Monica and Berkeley, evidenced by the issuance of few housing permits, as a driver of high housing costs. Those costs discourage minority middle-class households “who have not built up housing equity (because their parents were unlikely to be home owners),” Kahn e-mailed.