A study from 1998 that followed patients released from psychiatric hospitals found that they were no more prone to violence than other people in their communities—unless they also had a substance abuse problem. So mental illness alone was not a risk factor for violence in this study.

Those are the facts. But cultural narratives are often more powerful than facts, and that 4 percent gets overblown in people’s minds.

A new study published in Health Affairs shows how the news perpetuates this narrative, with a look at how several prominent newspapers and broadcast networks covered mental illness from 1995 to 2014. More than half of the stories they looked at during that period—55 percent—mentioned violence in conjunction with mental illness. That proportion was pretty much consistent across the 19 years. But stories connecting mental illness with mass shootings specifically increased from 9 percent between 1994 and 2004 to 22 percent between 2005 and 2014.

Perhaps this can be partially attributed to high-profile shootings like the Tucson shooting in 2011, in which the killer did have schizophrenia. “That’s an event that is newsworthy, but the fact that it was linked to mental illness is not representative of most people who have schizophrenia, or most violence,” says Emma McGinty, the lead author on the study and a professor of health policy at Johns Hopkins University. “[And yet] that link pervades the public psyche.”

It pervades so much so that people speculate about killers’ mental states, even in the absence of any evidence that they were living with any disorder. For example, in an article about the gunman who recently killed a professor at the University of California, Los Angeles, New York magazine writes: “Police do not know for sure yet if Sarkar had a history of mental illness.” Why does this particular absence of information bear mentioning? It seems mental illness is so linked to gun violence in people’s minds that we have to address it even when it’s not there.

And when there is evidence that a killer also happened to have a mental illness—like the pilot who crashed a Germanwings plane in 2015, who had a history of depression—the media seize upon it like a bear trap. “We’ve got it now! This is what was wrong with him,” is the message portrayed.

This is a really tricky needle to thread, because something was clearly wrong with him. Of course someone who is perfectly healthy and well-adjusted in every way would not go out and kill a bunch of people.

“This is one of the hardest distinctions to make,” McGinty says. “Anyone who kills someone else in a mass shooting scenario or otherwise is not what we would consider mentally healthy. But that does not mean they have a clinical diagnosis and therefore a treatable mental illness. There could be emotional regulation issues related to anger, for example, which are a separate phenomenon. There could be underlying substance use issues. There could be a whole host of other risk factors for violence going on.”