There's a surprisingly strong link between mental health and gun violence, but it probably isn't what you think

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David Hogg, a senior at Marjory Stoneman Douglas High School, speaks at a rally calling for more gun control three days after the shooting at his school, in Fort Lauderdale, Florida on February 17, 2018.

• US politicians like President Donald Trump have blamed recent deadly shootings on individuals' mental health problems.
• But mental health issues aren't predictive of violent outbursts: Although one in five Americans struggle with mental illness, people with mental health problems account for just 3% of violent crime.
• There is a different, notable link between violence and mental illness: People with major mental illnesses are 2.5 times more likely to be the victims of violent outbursts than the general public.

As mass shootings by young, angry men armed with machine guns become deadlier and more frequent in the US, politicians like President Donald Trump have suggested that mental health problems may be fueling these violent massacres.

"So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behavior," Trump tweeted after the deadly shooting in Parkland, Florida that killed 17 people. "Neighbors and classmates knew he was a big problem. Must always report such instances to authorities, again and again!"

Although reporting worrisome behavior is a good idea, the scientific evidence about the connection between violence and mental health issues doesn't support Trump's position.

Nikolas Cruz, the 19-year-old former student who shot up his Florida high school last week, had a reportedly checkered history of jealous and violent outbursts - but that's not the same thing as a bona fide mental health diagnosis.

Mass shooters don't usually have diagnosed mental health issues

In 2015, Psychiatrist Michael Stone catalogued a comprehensive database of more than 235 mass murders committed in the US. He found that in reality, about a quarter of the perpetrators of those acts were "clearly mentally ill."

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People hug as they attend a candlelight memorial service for the victims of the shooting at Marjory Stoneman Douglas High School that killed 17 people on February 15, 2018 in Parkland, Florida.

But the majority of the rage-filled, bigoted, grudge-holding men who plan these kinds of tragic killings don't necessarily mentally ill, Stone said.

Stone said many people assume that because someone has committed a deadly act, that must mean they're crazy. But that's not true.

In fact, according to the American Psychiatric Association, people with serious mental health problems account for just 3% of all violent crime, though as many as one in five people in the US experience a mental illness every year.

Most mentally ill people are never violent. Information from the National Center for Health Statistics similarly shows that fewer than 5% of the 120,000 gun-related killings in the US between 2001 and 2010 were perpetrated by people diagnosed with mental illness. Only 1% of discharged psychiatric patients commit violence against strangers using a gun. And the mentally ill also account for less than 3% of all violent acts with guns.

Data from American Psychiatric Association suggests Americans are about 15 times more likely to be struck by lightning than to be killed by a stranger with a mental diagnosis.

According to Stone's research, even shooters who are mentally ill aren't typically on anyone's radar before they act. Three of the most dramatic mass murders by people with diagnosable mental illness in recent history include the shooting at Sandy Hook that killed 27 (Adam Lanza), the movie-theater shooting in Aurora, Colorado that killed 12 in 2012 (James Holmes), and the six people who died in Tucson when Rep Gaby Giffords was shot in 2011 (Jared Lee Loughner).

The perpetrators of these deadly crimes were all "young men, barely 20, with no record of previous mental hospitalizations and no compelling reason why they should not have been permitted to buy rifles," Stone said.

Of course, mental illness often goes undiagnosed. Estimates suggest it's actually more common in developed countries than any other group of illnesses, including cancer and heart disease. According to the Centers for Disease Control and Prevention, roughly 80 million American adults are struggling with a mental illness at any given time, which is a far cry from the 19.2 million that successfully seek treatment every year. The American Psychological Association estimates that only about a third of depressed patients in the US ever get diagnosed by their primary care doctors.

But given that mental health problems are more common among women than men, and just as common in the US as all other highly developed countries, mental health is not a logical explanation for America's mass shooting problem.

People with mental health problems are more likely to be victims

Experts say that legislation restricting mentally ill people from getting guns would not do much to stop the deadly carnage the US now sees on a regular basis.

But there is a link between mental health issues and violence. Time and again, studies have shown that mentally ill patients are two to three times more likely to be the victims of violent crime than other people.

If someone has been diagnosed with a serious mental illness, like bipolar disorder or schizophrenia, that number jumps to 12 times the normal rate.

Researchers suggest that people struggling with mental illnesses might face an increased risk for victimization because they tend to live in more dangerous places (like on the streets or in group homes), deal with drug and alcohol addiction, or become irritable, paranoid, and less aware of their surroundings.

If mentally ill people aren't committing mass murder, who is?

People who study violent events say there is a well-established pattern among most mass shooters: They're typically angry young men who feel they've been "wronged" and are looking for revenge.

Forensic psychiatrist Liza Gold teaches at Georgetown and edited the book "Gun Violence and Mental Illness." She told Business Insider in 2017 that mass shooters tend to be "impulsive and angry about a lot of different things" and many have a history with law enforcement or violence, especially domestic violence.

Overall, the ratio of male killers to female killers in Stone's cataloged, which dates back to 1913 in the US, is 24 to 1. Stone says that makes sense psychologically, since men have been shown to be more likely to take out their aggression and anger on the outside world. Women, on the other hand, tend to be more "intropunitive" and blame themselves for their anger.

The men who go on killing sprees also tend to be young (roughly 85% are under the age of 44) and working-class. A 2003 study in the journal World Psychiatry summed the profile up this way: "the major determinants of violence continue to be socio-demographic and socio-economic factors such as being young, male, and of lower socio-economic status."

That's not something that can be alleviated with more mental health treatment, but it is a worrisome trend that seems likely to continue if young, rageful American men continue to have easy access to guns.