The murder rate among black teens has climbed since 2000 even as murders by young whites have scarcely grown or declined in some places, according to a report.

The celebrated reduction in murder rates has concealed a "worrisome divergence," said James Alan Fox, a criminal justice professor at Northeastern University who wrote the report, to be released today, with Marc L. Swatt. They said the gap could grow without such countermeasures as restoring police officers in the streets and creating social programs for poor youth.

The main racial difference involves juveniles ages 14 to 17. In 2000, 539 white and 851 black juveniles committed murder, according to an analysis of federal data by the authors. In 2007, the number for whites, 547, had barely changed, while that for blacks was 1,142, up 34 percent.

The report lays primary blame on cutbacks in federal support for community policing and juvenile crime prevention, reduced support for after-school and other social programs and a weakening of gun laws. Cuts in these areas have been felt most deeply in poor, black, urban areas, helping to explain the growing racial disparity, Fox said.

But Harvard sociologist Bruce Western said the change in murder rates was not large and did not yet show a clear trend. Western also said the impact of the reduction in government spending would have to be studied on a city-by-city basis, and that many other changes, including a sagging economy, could have affected murder rates.

NEW YORK TIMES