The number of Americans living in communities of extreme poverty — neighborhoods in which at least 40 percent of the population is poor — soared by one-third between 2000 and the latter half of the decade,according to a new study from the Brookings Institution.

The marked increase in so-called concentrated poverty underscores the distress tearing at communities across the nation amid the worst economic downturn since the Great Depression. It highlights a stunning reversal of economic fortune since the 1990s, when powerful job growth combined with the expansion of tax credits for lower-income households lifted millions of Americans above the poverty line.

Between 1990 and 2000, the number of poor people living in concentrated areas of poverty plunged from 4.4 million to three million, according to the study. By 2009, the number again exceeded four million, and the Brookings researchers assume the figure will be larger still when the Census releases detailed data for 2010. Preliminary figures for 2010 showed more than 46 million Americans — some 15 percent of the population — living below the federal poverty line, defined as annual income of $22,314 for a family of four.

“The gains that we made in the 1990s, with targeted policies and a booming economy, a lot of those have been erased over the 2000s,” said Elizabeth Kneebone, a senior research associate at Brookings’ Metropolitan Policy Program, and the study’s lead author. “Places that used to be solidly working class in the ’90s have fallen behind after two recessions.”

The broad elimination of working opportunities in many poor communities has left millions of people effectively stranded on islands of economic desolation, with the attendant problems of poverty — dilapidated housing, crime, social strife — deterring the investment that might alleviate their plight.

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