There may be as many explanations as there are killers. But over the decades, scientists have seen some patterns emerge.
On Monday morning, President Donald Trump made his first televised statement about the mass murders committed over the weekend in El Paso, Texas, and Dayton, Ohio. He called for action to "stop mass killings before they start," citing what he said were a number of 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. 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 boy who slaughtered elementary school children and teachers in Sandy Hook, Connecticut, had studied the Columbine massacre, among many others. The man who shot to death 50 people at the Pulse nightclub in Orlando, Florida, had studied a previous attack, in San Bernardino, California. In both cases, the murderers cited radical Islam as justification.
The young man accused of shooting to death more than 20 adults and children in a Walmart in El Paso over the weekend had seen the video posted by the man who gunned down unarmed worshippers at mosques in Christchurch, New Zealand.
Forensic psychologists say that many would-be mass killers see themselves as part of a brotherhood of like-minded, isolated and resentful boys and men. To them, previous mass murderers may be perceived as idols and pioneers.
Are video games to blame for mass shootings?
The results of studies attempting to clarify the relationship between violent video games and aggression have been mixed, with experts deeply divided on the findings. A just-published analysis of the research to date concludes that "in the vast majority of settings, violent video games do increase aggressive behavior" — but that "these effects are almost always quite small."
The "aggression" in question falls well short of assault with a weapon, never mind mass murder. So the weight of scientific opinion is that video games are not a decisive factor when a spree killer decides to act.
Establishing a persuasive link between shooting digital figures from the couch and real people in a mall or school is a long shot. A huge proportion of males in the United States have played or are playing video games; only a handful commit mass murders. And video games are even more popular in Asian countries, where mass killings are far rarer.
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How strong is the link between mental illness and 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 characterised 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, Arizona, hosted by then-Rep. Gabrielle Giffords in 2011, killing six and wounding 13. Loughner's online posts demonstrated increasing drug use and paranoid fantasies.
About 1 in 5 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 per cent have many of the problems that nearly everyone has to manage at some point in life: anger, isolation, depressive moods, resentments, jealousy.
Would drugging or confining people showing "red flags" prevent massacres?
No one knows for certain. In his speech, Trump mentioned the teenager who in 2018 killed 17 people at a high school in Parkland, Florida. 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.
Red-flag policies, tracking threats and other signs of trouble, have been in place for years in some school districts around the country. Would-be shooters often reveal their intentions in dark asides or rants online. They may profess respect for past mass killers even as they stockpile weapons and ammunition.
Los Angeles County, in particular, has intervened in scores of such cases since its program was implemented in 2007. It has not had a major school shooting, though there is no way to know if the program has prevented any.
Still, such preventive measures get students into therapy, and alert parents and teachers to warning signs: They do not require forced drugging and confinement before any crime has been committed. Implementing that kind of policy would require a thoughtful reconsideration of individual rights in this country — which, given partisan gridlock, is not likely to happen.
Written by: Benedict Carey
Photographs by: Jim Wilson
© 2019 THE NEW YORK TIMES