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The housing crisis was also a major migration event, although we seldom think of it that way. As many as 10 million families lost their homes to foreclosure. As a result, nearly all of them them had to move.

They didn't make for the kind of dramatic cross-country moves that have characterized other great population shifts in U.S. history — west in search of open land, or north in hopes of factory work. For the most part, nearly all of these foreclosed families stayed within the same metropolitan area. But the sheer scale of all of their short-distance reshuffling has had a real and largely hidden impact in cities: It reversed much of the recent progress we've made on racial integration.

According to new research, migration patterns set in motion by the foreclosure crisis slowed declines in segregation across metropolitan America between blacks and whites by 19 percent, and between whites and Hispanics by 50 percent. It's well-documented that minorities were hit particularly hard by foreclosures. But this work, published in the American Sociological Review, suggests that those racial disparities were compounded by what happened next: Minorities who lost their homes moved to more distressed neighborhoods, while white homeowners who could leave appear to have been the first to pull out of places hit by foreclosure.

Over time, these parallel trends made many neighborhoods more segregated than they likely would have been if the housing crisis had never happened. The most troubled neighborhoods in particular grew less white and more heavily minority. And these changes in individual neighborhoods added up so that segregation increased during this time across whole metropolitan areas.

"My general worry is that the progress we’ve been making toward racial integration has been partly derailed," says Cornell's Matthew Hall, who conducted the research with Kyle Crowder at the University of Washington and Amy Spring at Georgia State.

Racial segregation between blacks and whites in the U.S. has been declining for decades, but very gradually, and in some places less so than others. The recent uptick in segregation that Hall, Crowder and Spring measured is small but still significant. It was also particularly large in Western cities heavily hit by the housing bubble like Las Vegas and Sacramento.

"Segregation is very sticky, it’s extremely slow to change," Hall says. "The changes that we have across decades tend to be pretty small, so the effect that we’re picking up is pretty large in that context."

The study also uncovered another particularly alarming finding: Foreclosure rates during the housing bust were highest in the most integrated neighborhoods. So not only were we becoming more segregated — but the least segregated places were heavily undermined by foreclosure at the same time. The authors aren't entirely sure why this would be, but the finding speaks to the relative fragility of rare, integrated communities.

The scale of this entire foreclosure migration is deceptively large. The 10 million households that lost their homes dwarf the number that left the Great Plains during the Dust Bowl (that was about 2.5 million people). In fact, it's larger than the 6 million blacks who moved north during the Great Migration — a movement that spanned decades. That number doesn't even include all the households that never went through foreclosure but moved anyway because many of their neighbors did.

If such a sizable population shift nudged us — albeit a small amount — toward a more segregated society, that bodes poorly for all the work that has to happen to head back in the other direction.

"Segregation changes almost entirely via migration," Hall says. "In order to have long-term impacts on segregation, you have to somehow alter the migration patterns of racial groups."