As newly middle-class minorities moved to inner suburbs, though, the mostly white residents of those suburbs moved further away, buying up the McMansions that were being built at a rapid pace. This acceleration of white flight was especially problematic in Rust Belt towns that didn’t experience the economic boom of the mid-2000s. They were watching manufacturing and jobs move overseas.

Population Living in High-Poverty Neighborhoods (in millions)

Source: 1990 and 2000 Census, 2005-2009 and 2009-2013 ACS/The Century Foundation

Cities such as Detroit saw continued white flight as wealthier residents moved to Oakland County and beyond, further and further away from the city’s core. They brought their tax dollars with them, leaving the city with little tax base, a struggling economy, and no resources to spend on services.

Low-income residents who wanted to follow the wealthy to the suburbs would have had a difficult time. Many wealthy suburbs passed zoning ordinances that prohibited the construction of affordable-housing units or the construction of apartment buildings in general. Some mandated that houses all be detached, or are a minimum size, which essentially makes them too expensive for low-income families.

“It’s no longer legal to say, ‘We don’t want African-Americans to live here,’ but you can say, ‘I’m going to make sure no one who makes less than two times the median income lives here,’” Jargowsky told me.

(Though some affordable-housing developers try to build in the suburbs, many more, especially those in the “poverty-housing industry,” advocate for building more developments in high-poverty areas to stimulate economic growth. The Local Initiatives Support Corporation, which has a goal of investing in distressed neighborhoods, for example, has spent $14.7 billion building affordable housing units since 1980.)

Some of the cities where poverty is the most concentrated are in the Midwest and Northeast, where tens of thousands of people have headed to suburbs, and the region itself is shrinking in population. In Syracuse, New York, for example, 65 percent of the black population lived in high-poverty areas in 2013, up from 43 percent of the black population in 2000, Jargowsky found. In Detroit, 58 percent of the black population lived in areas of concentrated poverty in 2013, up from 17 percent in 2000. And in Milwaukee, 43 percent of the Latino population lived in areas of concentrated poverty in 2013, up from 5 percent in 2000.

The number of high-poverty census tracts is also growing in many of these cities. In Detroit, the number of such tracts tripled to 184, from 51 between 2000 and 2013, as concentrated poverty spread to inner suburbs. In Syracuse, the number of high-poverty census tracts grew to 30 from 12.