What Drives People to Mass Shootings?

Tenuous, at best. People who blame mass shootings on “the mentally ill” are usually reasoning backward from the act itself: the person just shot 20 unarmed strangers, so he must be “crazy.”

In fact, scientists find that only a small fraction of people with persistent mental distress are more likely than average to commit violent acts: patients with paranoid schizophrenia, which is characterized by delusional thinking and often so-called command hallucinations — frightening voices identifying threats where none exist.

People living in this kind of misery are far more likely to be the victims of violence than perpetrators; but they can act violently themselves, especially when using drugs or alcohol. The clearest recent example is Jared Loughner, the college student who opened fire at an event in Tucson, Ariz., hosted by former Rep. Gabrielle Giffords in 2011, killing six and wounding 13. Mr. Loughner’s online posts demonstrated increasing drug use and paranoid fantasies.

About one in five mass murderers shows evidence of psychosis, according to Dr. Michael Stone, a forensic psychiatrist who maintains data on some 350 murderers going back more than a century. The other 80 percent have many of the problems that nearly everyone has to manage at some point in life: anger, isolation, depressive moods, resentments, jealousy.

No one knows for certain. In his speech, Mr. Trump mentioned the teenager who in 2018 killed 17 people a high school in Parkland, Fla. It’s a good example: Before his murder spree, the shooter talked of his intentions to such an extent that classmates joked that he was the student most likely to shoot up the school.

“Unfortunately, it is wishful thinking to believe that there is a simple set of warning signs, a phone app or a checklist which can be used to identify a mass shooter,” said Dr. Deborah Weisbrot, director of the outpatient clinic of child and adolescent psychiatry at Stony Brook University.

She has interviewed about 200 young people, mostly teenage boys, who have made threats.

“There is no specific ‘profile’ of a shooter, as is still often sometimes assumed — there have been both male and female shooters, and different socioeconomic backgrounds,” she said.