Cities breed their own pathologies, and homicide is a predominantly urban form of lawlessness. But mass murder, in fact, is not. When James Alan Fox, a criminologist at Northeastern University who has written extensively about extreme killing, compiled homicide data from across the country for 1976 to 2010, he found that 64 percent of mass murders — defined as any instance in which four or more people were killed more or less at the same time — had occurred in places with populations of less than 250,000