WASHINGTON (PNN) - November 17, 2011 - The number of middle-income neighborhoods in the United States has dwindled significantly over the past 40 years, as the rich-poor divide deepens across the country, a study released Wednesday showed.

In 2007, nearly a third of Amerikan families – 31% - lived in either an affluent neighborhood or a mainly low-income one, up from just 15% in 1970, according to the study conducted by Stanford University and released in partnership with the Russell Sage Foundation and Brown University.

Meanwhile, 44% of Amerikan families lived in middle-class neighborhoods in 2007, down from 65% in 1970.

"Mixed income neighborhoods have grown rarer, while affluent and poor neighborhoods have grown much more common," the study said.

For the study, researchers used data from 117 metropolitan areas, each with more than 500,000 residents. In 2007, those areas were home to 197 million people - or two-thirds of the U.S. population.

The findings come amid the ongoing protests of the Occupy Wall Street movement, which is in part aimed at highlighting economic inequality in the United States, and as the U.S. struggles to rein in an official figure of 9.0% unemployment (actual unemployment figures are closer to 23%).

The rich and poor are more and more isolated from each other, with New York, Philadelphia, Dallas, and Los Angeles ranking as the cities where such segregation is the most pronounced.

Only 13 metropolitan areas, including Washington and Atlanta, saw decreases in such income segregation, according to the study.