So is mental illness to blame for America’s mass shootings? Not according to research.

Some mass shooters have a history of schizophrenia or psychosis, but many do not. Most studies of mass shooters have found only a fraction have mental health issues.

Researchers have noted a host of other factors that are stronger predictors of someone becoming a mass shooter: a strong sense of resentment, desire for infamy, copycat study of other shooters, past domestic violence, narcissism and access to firearms.

“It’s tempting to try to find one simple solution and point the finger at that,” said Jeffrey Swanson, a professor in psychiatry and behavioural sciences at Duke University School of Medicine.

“The fact that somebody would go out and massacre a bunch of strangers, that’s not the act of a healthy mind, but that doesn’t mean they have a mental illness.”

As mass shootings have become more common in recent years, their connection to mental health has been increasingly scrutinised by the Federal Bureau of Investigation, police departments, forensic psychiatrists, mental illness experts and epidemiologists.

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In a 2018 report on 63 active shooter assailants, the FBI found that 25 per cent had been diagnosed with a mental illness. Of those, only three had been diagnosed with a psychotic disorder.

In a 2015 study that examined 235 people who committed or tried to commit mass killings, only 22 per cent could be considered mentally ill.

Research has long debunked another common explanation touted by politicians: that violent video games are driving the mass shooting crisis, an idea floated again by Trump and House Minority Leader Kevin McCarthy, on Monday.

“We must stop the glorification of violence in our society,” Trump said.

“This includes the gruesome and grisly video games that are now commonplace. It is too easy today for troubled youth to surround themselves with a culture that celebrates violence. We must stop or substantially reduce this, and it has to begin immediately.”

It’s true that some recent mass shooters played violent video games obsessively.

Adam Lanza, who killed 26 schoolchildren and school employees at the Sandy Hook Elementary School in Connecticut in 2012, spent hours a day playing some of the most violent video games, including one called School Shooting.

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There is, however, no statistical link between playing violent video games and shooting people, said Jonathan Metzl, director of the Centre for Medicine, Health and Society at Vanderbilt University, who has studied the topic.

A 2004 report conducted by the Secret Service and the Education Department found that only 12 per cent of perpetrators in more than three dozen school shootings showed an interest in violent video games.

And hundreds of millions of people around the world play first-person shooter games such as Fortnite and do not turn into mass murderers.

Despite a continuing lack of a link, lawmakers and public figures continue to blame the gaming industry.

“When politicians like President Trump perpetuate this narrative, to me, it is the height of irresponsibility because it’s perpetuating a falsehood,” Metzl said.

The eagerness to blame mental health and video games means society is searching for answers in the wrong places, experts say.

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At the height of the reflex to blame video games – following the Columbine high school shooting in 1999 – a Gallup poll found that 62 per cent of adults nationwide believed entertainment was the major catalyst for the tragedy and that 83 per cent supported restrictions on the sale of violent media to children.

US President Bill Clinton even called for an investigation on how the advertising industry sold violent entertainment. Last year, a Washington Post-ABC poll on mass shootings found that 57 per cent of people believed shootings were a reflection of failures to identify and treat people with mental health problems.

Meanwhile, only 28 per cent thought it reflected inadequate gun control laws.

“The irony is clearly we need more robust mental health system,” said Arthur Evans Jnr, a psychologist who heads the American Psychological Association.

“But that’s separate and apart from these shootings.”

Almost 5 per cent of the US population suffers from a serious mental illness in a health care system that most clinicians say severely under-prioritises mental health.

That has often left psychiatric wards without enough beds for those who are in crisis.

People with serious mental disorders are 3.6 times more likely to exhibit violent behaviour, according to the National Epidemiologic Survey on Alcohol and Related Conditions.

But they are far more likely to be the victims of violence – with a 23 times higher risk compared with the general population.

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A study published in the journal Annals of Epidemiology found that “the large majority of people with mental disorders do not engage in violence against others, and that most violent behaviour is due to factors other than mental illness.”

“We like to think that anyone who kills others is somehow mentally ill,” said Phillip Resnick, who served as a forensic psychiatrist in cases including Oklahoma City bomber Timothy McVeigh and Unabomber Ted Kaczynski.

“But you have to remember, people kill for all sorts of reasons. They kill for profit or love or greed.”

Marine Corps veteran Ian David Long, 28, who killed 12 at a country music bar in California in November last year, was believed to have post-traumatic stress disorder.

And Cruz, the Florida high school attacker, had a history of mental health problems.

Some attacks are also linked to divisive politics, which play out online, especially on the 8chan website.

Like two earlier mass murderers, the man who killed 22 at a Walmart in Texas on Saturday, identified in media reports as 21-year-old Patrick Crusius, released a “manifesto” on 8chan attacking the “Hispanic invasion” from Mexico, echoing Trump’s own political rhetoric.

Mental stress and emotional disturbance can be a factor in a mass shooting. The 2018 FBI study found that shooters typically experienced several stressors in the year before they attack – financial pressures, fights with classmates or colleagues, and substance abuse.

And on average, gunmen displayed four to five concerning behaviours that those around them could notice – the most frequent being behaviour related to mental health, interpersonal conflicts or some sign of violent intent.

“These may be angry, alienated, troubled young men who are marinating in hate for some other group, for example, and have access to this extremely lethal technology,” Swanson said.

“So to me, saying it’s mental illness is a big dodge to not talk about guns.”

Additional reporting by Agence France-Presse