Moreover, much of the violence that people with serious mental illnesses commit are minor infractions, such as verbal assault or hitting, not homicide (suicide, however, is a significant problem), and such infractions tend to be directed at those the perpetrator lives with, not at strangers and not at a mass scale. Large attacks also require a level of planning and organisation that often defies many with serious mental illness. A 2014 study, for example, found that just 2% percent of 951 patients discharged from a psychiatric hospital committed a violent act involving a gun, and just 6% committed a violent act involving a stranger.

A 2011 meta-analysis of over 700 homicides committed by people with a diagnosed psychosis likewise revealed that just between 3% and 14% of the victims were strangers – the rest were known to the killer.

But even in cases in which people with a serious mental illness do become violent, it’s difficult to peg their crimes solely on their diagnosis, Swanson says. Confounding variables such as a history of childhood abuse or use of alcohol or drugs can increase the odds of violence. “If you remove additional risk factors and look at mental illness alone, it’s almost unrelated to violence,” Swanson says. “There’s consensus in the field that the unique contribution of psychopathology to violence in the population is very small.”

In the wake of an incomprehensible tragedy, however, the implications of that conclusion can be difficult for many to process. “When one of these horrible mass shootings occurs, people say, ‘Anyone who would do such a thing must be mentally ill,’” says Renee Binder, a professor and director of the psychiatry and law program at the University of California San Francisco School of Medicine. “But we need to be careful with our definitions because, while something is clearly wrong with them, it’s often not a serious mental illness.”