That wasn't always the case. A generation ago, the region was a model for the post-World War II, middle-class lifestyle. High-wage manufacturing jobs were abundant, particularly in the aerospace industry. When the industry collapsed in the early 1990s, many middle-class residents left the region. In the meantime, large numbers of immigrants arrived seeking work.
Other changes mirrored national trends, including the development of large, similarly priced housing tracts outside city cores.
Now even the suburbs are growing apart. In a study conducted two years ago, Dreier and three colleagues found an increasing polarization of the rings surrounding U.S. cities.
"There are a growing number of wealthy suburbs, a growing number of poverty suburbs and an absolute decline in the number of middle-class suburbs," he said.
Los Angeles' suburbs also were among the nation's most extreme. Only suburbs of Phoenix and Palm Beach, Fla., were more polarized, the researchers found.
Both reports on geographic polarization were released by the Washington, D.C.,-based Brookings Institution.
Alan Berube, a Brookings urban affairs specialist, said the pulling apart of rich and poor has immediate and tangible effects. For one thing, it can diminish choices and raise prices for everyone.
"The retailers in the two neighborhoods are very different," he said. "It's the difference between a Whole Foods and a corner grocer, or Citibank and the local check casher. They're not competing, and in the end, you have higher prices for all basic goods and services."
More broadly, Berube maintained, the shrinking of mixed or middle-income neighborhoods limits the ability of low-income residents to move up economically without leaving the region. It can even contribute to civic instability.
"People in the middle are an important social glue for a city," he said. "To some extent, they serve to mediate the interests of low-income versus high-income, to help forge compromise."
Dreier and Galster said government intervention is needed to reverse the trend.
In metropolitan regions that are continuing to grow, such as Los Angeles, they advocated a requirement that developers build a mix of housing, including affordable units for low-income residents.
Known as "inclusionary" housing, that tool has been adopted by dozens of California cities. But in Los Angeles it has been successfully opposed by business groups that have argued it would discourage developers from building in the city at all.
