A neighborhood of concentrated poverty in Baltimore. (Linda Davidson / The Washington Post)

Despite their ubiquity in the media, gentrifying neighborhoods that evolve over time from low-income to well-off are quite rare. It is far, far more common that once-poor neighborhoods stay that way over time — or, worse, that they grow poorer.

Joe Cortright and Dillon Mahmoudi illustrate this plainly in a new City Observatory analysis tracking the persistence of poverty over the last four decades in the 51 largest metropolitan areas in the U.S. Back in 1970, there were 1,100 high-poverty Census tracts in the nation's largest cities (these are places where the poverty rate tops 30 percent).

By 2010, a mere fraction of those same Census tracts — about 100 of them in all — had poverty rates that had fallen below the national average.

The overwhelmingly majority of these urban neighborhoods that were poor in 1970 have not seen new condo construction, or population growth or socioeonomic change in the years since then. They have seen, rather, persistent poverty, and the lingering disadvantages that come with it.

What's more, the number of such high-poverty urban Census tracts in urban America has proliferated since the 1970s. There were 1,119 in 1970, by Cortright and Mahmoudi's count (they've looked at tracts within 10 miles of the central business district in each of these 51 metros). By 2010, there were 3,165.

In that time, the total population living in high-poverty census tracts has more than doubled, as has the number of poor people living there in concentrated poverty:

According to Cortright and Mahmoudi, this spread and persistence of concentration of poverty — and not gentrification — is the "biggest problem confronting American cities." And these trends touch nearly every city in their analysis. Just a sampling of them: