New York’s middle class is shrinking as it is in other cities across the nation and the city’s middle-income neighborhoods are also declining in number. An analysis of census figures by the Washington-based Brookings Institution attributed the change to a renewal of marginal neighborhoods and a tendency by high-income families to live together, according to The New York Times. The analysis also showed that the number of middle-income neighborhoods nationwide declined from 58 percent in 1970 to 41 percent in 2000.
http://www.nytimes.com/2006/06/22/nyregion/22income.html