Mental health authorities, however, disagree. They say there have been some mass shooters who clearly are mentally ill -- for example, Jared Lee Loughner, a diagnosed schizophrenic who killed six people in a Tucson shopping center in 2011 and severely injured 13 others, including U.S. Rep. Gabrielle Giffords, (D-Ariz). But they're in the minority. More often, experts say, the gunmen have less severe personality or emotional problems that are common in the population at large. Instead of mental illness, it's other factors -- such as a prior history of violence, extreme beliefs, and access to firearms -- that are better predictors of who will commit mass murder.
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"Mental illness is only weakly, weakly associated with gun violence," says forensic psychiatrist Dr. Liza H. Gold, a professor at Georgetown University.
Dr. Michael Stone, a professor of clinical psychiatry at Columbia College of Physicians and Surgeons in New York City, has amassed data on more than 300 mass killers, and he's come to a similar conclusion. Stone says only about 18 to 20 percent of mass killers have serious mental illnesses, such as schizophrenia or bipolar disorder. More often, he says, they're simply men who are disgruntled. "They're paranoid, mistrustful, not very successful, not getting too far in life," he says. "They suffer a reverse, such as being rejected by a boss or a lover, and that usually is what sets them off."
That's not to say that mass killers are mentally healthy people. One study found that 60 percent experienced symptoms such as paranoia or depression. But Dr. Jonathan M. Metzel, a psychiatrist and sociologist who heads the Center for Medicine, Health and Society at Vanderbilt University and has co-authored a 2014 survey of scientific research and news media information on mass killers, explains that that such symptoms aren't necessarily proof that mental illness led them to engage in slaughter.
"When we get into a causal explanation, that's where we have problems," he said. "It's hard to predict who actually is going to shoot someone. Thousands of people might have those symptoms, but only one might commit such an act."
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In the survey article, Dr. Metzel and co-author Kenneth T. MacLeish noted that fewer than 5 percent of the 120,000 gun-related killings between 2001 and 2010 were committed by people diagnosed with mental illness. A 2014 study by North Carolina State researchers found that adults with mental illness were more likely to become victims of violence than perpetrators.
Several experts cautioned against jumping to the conclusion that Orlando killer Mateen was mentally ill, based just upon the limited amount of information available about him so far. Dr. Gold says that it seemed more likely that Mateen's murderous behavior, in the absence of mental illness, was shaped by a propensity for anger and fanatical beliefs. If Mateen himself was secretly gay, as some reports now suggest, that could have thrown more fuel on the fire, she says.
"With a real mental disorder, you'd see other signs of deteriorating mental function," Dr. Gold says.
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If a killer is involved with like-minded fanatics, either in person on or the internet, that also is an indication that mental illness isn't the driving factor, she explains. "Mental illness usually isn't shared, but fanaticism requires other people," she says.
Experts also say that even if some killers turn out to be mentally ill, better mental healthcare isn't necessarily the most effective way to prevent more slaughter. "It makes more sense to limit the means of killing," such as restricting access to military-style weaponry, Dr. Metzel says.
A recent study by Mayors Against Illegal Guns, an advocacy group, found that when states submit mental health records to the FBI for use in background checks on gun purchases, the number of individuals prevented from getting firearms increased by 65 percent over a three-year period.