On Monday morning, President Trump made his first televised statement about the mass murders committed over the weekend in El Paso, Tex., and Dayton, Ohio. He called for action to “stop mass killings before they start,” citing what he said were a number contributing factors: the contagious nature of mass murder; the glorification of violence in video games; and the need to act on “red flags” to identify and potentially confine the “mentally ill monsters” that he said commit the crimes.

Many of these factors have been studied by scientists for decades. Here are answers to some of the most common questions about the causes of mass murder.

Can one mass shooting inspire another?

Yes. The police find abundant evidence that shooters have studied previous crimes, often mimicking gestures or killing tactics, as if in homage to previous killers. This is true both of younger shooters who mow down unarmed people in schools, or at random; and of older men who execute innocents in the name of an ideology — be it opposition to immigration, white supremacy, radical Islam or another extreme belief.

The young man who slaughtered elementary school children and teachers in Sandy Hook, Conn., had studied the Columbine massacre, among many others. The man who shot to death 50 people at the Pulse nightclub in Orlando, Fla., had studied a previous attack, in San Bernardino, Calif. In both cases, the murderers cited radical Islam as justification.