According to the National Institutes of Health, people with a mental health diagnosis committed less than 5 percent of gun-related homicides between 2001 and 2010. Mental health experts, including representatives from the American Psychological Association, have called it “unfounded” to blame mass shootings on mental illness in place of considering other possible factors, such as hate, bigotry, and access to assault weapons.

Calling every mass shooting a mental health problem is “inaccurate and it’s stigmatizing,” said Arthur Evans, chief executive officer of the American Psychological Association.

Extensive case history shows that mass shooters don’t just suddenly break — they decide. They develop violent ideas that stem from entrenched grievances, rage, and despair. In many cases they feel justified in their actions and regard killing as the sole solution to a problem. They arm themselves and prepare to attack, choosing where and when to strike. Often this is a highly organized and methodical process.

Mental illness affects millions of adults across the country. About 1 in 5 adults in the United States, or 46.6 million, experience mental illness in a given year, according to the National Alliance on Mental Illness.

Based on the current body of research, mass shootings tend to fall within specific categories, most of which do not explicitly involve mental illness, according to Post, who has been tracking these “typologies” of massacres.

Although they seem like they happen all the time, public mass shootings are rare events and tend to have different motivations, said Jeffrey Swanson, a professor in psychiatry and behavioral sciences at Duke University, who specializes in gun violence and mental illness.

“They range from this extreme anger and hate to rare manifestations of acute psychopathology,” he said. “The risk factors for this kind of violence are non-specific.”

In 2018, a deep investigation of 63 rampage shooters conducted by experts with the FBI’s Behavioral Analysis Unit showed that only a quarter of the offenders were known to have been professionally diagnosed with a mental illness of any kind. While it’s possible that some suicidal attackers may have gone undiagnosed, only three of the 63 perpetrators, or about 5% of the total examined, had a known psychotic disorder.

Blaming mental illness for mass shootings inflicts a damaging stigma on the millions of people who suffer from clinical afflictions, the vast majority of whom are not violent. Extensive research shows the link between mental illness and violent behavior is small and not useful for predicting violent acts; people with diagnosable conditions such as schizophrenia or bipolar disorder are in fact far more likely to be victims than perpetrators of violence.