"The central cities of the large metropolitan areas are the epicenters of segregation," the Harvard study reported.

Philadelphia bears that out.

More than 30 years ago, the Pennsylvania Human Relations Commission went to Commonwealth Court to try to force a desegregation plan from the district. At the time, 70 percent of the city schools were racially isolated, either black or white.

The district dug in its heels at the commission's demand for mandatory busing. But since the early 1980s, when Constance Clayton became the city's first African American school superintendent, the district has bused tens of thousands of children in a voluntary program and poured millions of extra dollars into heavily minority schools.

Whatever integrating effect that might have had over the years has been all but neutralized - in part by white flight from the city, and in part by a parochial and private school tradition with few equals in the country.

Today, 169 of Philadelphia's 263 public schools, or about two-thirds, are almost exclusively minority. Two are 90 percent or more white. The Human Relations Commission lawsuit, still in Commonwealth Court, is no longer focused on desegregation but on resources for poor schools.

The story in the suburban ring is scarcely better.

Bucks, Montgomery, Delaware and Chester Counties have 63 districts and about 38,000 black students. One-third of those children are clustered in just three troubled, predominantly minority systems: Chester Upland and William Penn in Delaware County, and Norristown Area in Montgomery County.

The three area South Jersey counties - Burlington, Camden and Gloucester - are uncannily similar. Of their 45,000 black students, more than one-third are in three of the 102 districts: Camden, Willingboro and Winslow.

Ask why things are as they are, and the stock answer has been parental decision-making.

"The Northern argument was that people chose where to live based on market decisions, that it was a free market. It wasn't," said Tom Sugrue, a University of Pennsylvania historian and authority on race in urban America.

Often behind private decisions have been public policies and practices that have allowed and even encouraged segregation, particularly in Northeast states with many autonomous local governments. Pennsylvania has 2,566; New Jersey, 566.

When it came to zoning laws that kept out affordable housing by prescribing large house lots, or property tax systems that led to vast school-funding inequities, Brown did not level the playing field for blacks and whites.

"In many respects, government has acted at cross-purposes here," said Bruce Katz, a specialist in metropolitan growth for the Brookings Institution, a Washington think tank.

"On one hand it says, 'We don't want segregation in the schools.' On the other, it has almost dictated - through housing, infrastructure, land use - that there will be segregation."

For a couple of decades after Brown, activists argued to the courts that such policies were just less blatant Northern versions of the Southern laws that had been found unconstitutional. They had little success.

One case in particular closed off what many integrationists in big Northern cities considered the only remedy for segregation: the creation of regional districts.

J. Richardson Dilworth, Philadelphia's mayor from 1956-62 and then its school board president, maintained that the state needed to reconfigure the districts in the five counties. Otherwise, the suburbs would form a "white noose" around the city.

"Instead of these little suburban districts and this one massive Philadelphia district," he said in 1965, "we should set up about 12 districts in the area."

Milliken v. Bradley settled that. In the 1974 case, out of Detroit, the Supreme Court ruled the suburbs off-limits to the desegregation plans of cities where the schools had become majority-black - unless it could be proven that the government had deliberately created the segregation. The vote was 5-4, with Justice Thurgood Marshall among the dissenting justices.