Rising class segregation by residence is partly related to rising income inequality, but it is also the result of an expansion of exclusionary zoning. Research by Jonathan Rothwell, now at Gallup, and Douglas Massey, a sociologist at Princeton, has found that “a change in permitted zoning from the most restrictive to the least would close 50 percent of the observed gap between the most unequal metropolitan area and the least, in terms of neighborhood inequality.”

Exclusionary zoning frustrates the Fair Housing Act’s aim by erecting barriers that exclude millions of low-income African-Americans and Latinos from wealthier white communities. And growing numbers of poor whites are affected as well. As Paul Jargowsky, a professor of public policy at Rutgers University, notes, African-Americans and Hispanics remain much more likely to live in concentrated poverty than whites, but since 2000, there has been a 145 percent increase among non-Hispanic whites living in high-poverty neighborhoods.

Economic segregation matters because where you live affects so much in life — your access to transportation, employment opportunities, decent health care, and, most important, good schools. In Montgomery County in Maryland, which requires developers to set aside units for low-income families, disadvantaged students attending good local schools cut the math achievement gap with their middle-class peers in half between 2001 and 2007, according to a study by Heather Schwartz, a RAND Corporation researcher.

To bolster the 1968 Fair Housing Act, we need a new “economic fair housing act” to prohibit or discourage local ordinances that unnecessarily exclude people from entire neighborhoods and their schools. Given political realities in Washington, we could begin with laws in friendly states and build toward a moment, sometime in the future, when the federal government would take on the issue.

In its strongest form such a law would ban unjustified and pervasive exclusionary zoning laws that prohibit townhouses or apartments in single-family areas or impose minimum lot sizes. These ordinances, Lee Anne Fennell of the University of Chicago Law School notes, have become “a central organizing feature in American metropolitan life.”

If we can’t achieve a ban, we should assess a penalty on municipalities that engage in discriminatory zoning, either by withholding infrastructure funds or limiting the tax deduction that homeowners in those towns can take for mortgage interest. At the same time, inclusionary zoning laws of the type used in Montgomery County are needed to promote mixed-income housing.