IN THE LATE 1980s, as guns flooded the streets of many urban neighborhoods, there was a sharp rise in gang-related killings. In Boston, where gang violence reached crisis levels, most shootings occurred in three neighborhoods where gangs overlapped and fought over drug territory. Disputes that were once settled with fists were settled with bullets. These shootings also claimed the lives of anyone who got in the way. A boy interviewed for a television documentary on youth gangs describes the mood of violence in Boston's inner-city neighborhoods: "Anybody could shoot you, for the wrong look you could get shot. You could get shot for wearing Nike, you could get shot for anything."

To most outsiders, a gang appears to be a primarily destructive force. Though it provides a substitute family for many kids with troubled home lives, its so-called...