I BLAME THE MEDIA: Mass Killings May Have Created Contagion, Feeding on Itself. “The horrifying rash of massacres during this violent summer suggests that public, widely covered rampage killings have led to a kind of contagion, prompting a small number of people with strong personal grievances and scant political ideology to mine previous attacks for both methods and potential targets to express their lethal anger and despair.”

It’s called the Copycat Effect.