IN recent years, homicide rates in a number of large American cities have plummeted.  Between 1990 and 1996, New York’s rate dropped 58.7 percent, Houston’s 54 percent, Los Angeles’ 27.9 percent, Philadelphia’s 17.7 percent, and Washington, D.C.’s 15.9 percent.  In most, if not all of these cities, the precipitous decline in homicide rates derives from even sharper declines in youth violence. However, not all cities have been so fortunate. For example, in Baltimore, Phoenix, and Las Vegas, homicide rates have risen by 7.5 percent, 45.3 percent, and 103.8 percent respectively. A key question then is: Why has youth violence fallen so significantly in some cities but not in others?