In the aftermath of the United States’ latest mass shooting , political reactions diverged along well-trodden lines. For some, it was all about gun control . But for others, including President Donald Trump, the issue was the alleged shooter’s mental health.

So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behavi… https://t.co/3mSYrtTng6

The reality, according to experts who study the risk of gun violence, is more complex than the talking points on the right and left.

Yes, people with serious mental illness — including schizophrenia, bipolar disorder, and severe depression — are on average two or three times more likely to be violent than the average person. But it’s unclear how much of this is due to other problems, like substance abuse, that often to go hand-in-hand with mental illness. And overall, mental illness is thought to cause only about 4% of violent crime in the US.

So expecting better mental health treatment to solve America’s problems with gun violence is a forlorn hope. “It’s promising something that we can’t deliver,” Marcia Valenstein, a mental health services researcher at the University of Michigan, told BuzzFeed News.

At the same time, focusing on mental illness each time a mass shooting occurs hardens public attitudes against some of the most vulnerable people in society.

“To have people say that this person was deranged, or had a look in his eyes, doesn’t do anything other than create stigma,” Richard Van Dorn, a mental health services researcher at RTI International in Research Triangle, North Carolina, told BuzzFeed News.

Meanwhile, there are no easy answers to stopping mass shootings by trying to fix mental health:

We can’t predict who will commit acts of extreme violence.

It’s hard to ignore that many perpetrators of mass shootings have had a history of mental illness. The problem is differentiating these rare, dangerous individuals from the much larger number of mentally ill people who pose no real threat. As investigators tease apart the troubled past of Nikolas Cruz, 19, who is alleged to have shot and killed 17 students and staff at the Marjory Stoneman Douglas High School in Parkland, Florida, on Wednesday, there seem to have been many warning signs, including his expulsion from school and disruptive behavior at home.

But warning signs that seem obvious in hindsight are hard to spot in advance. What mental health professionals would need is a simple screening tool that provides reliable predictions of violence before people commit an atrocity.

That doesn’t yet exist. As news emerged of Cruz’s background, Van Dorn and his wife Sarah Desmarais pulled out a checklist called the Violence Risk Appraisal Guide, or VRAG. It asks questions about an individual’s background, including their school disciplinary record and prior violent offenses, and doesn’t require a face-to-face interview.

Van Dorn and Desmarais, a forensic psychologist at North Carolina State University, calculated a VRAG score that would predict a 17% chance that Cruz would be convicted of a violent crime over the next seven years.

“We don’t have a lot of faith in that number,” Van Dorn told BuzzFeed News.

The problem, according to Seena Fazel, a forensic psychiatrist at the Oxford University in the UK, is that screening tools like VRAG were developed to assess people already convicted of violent crimes, primarily to identify those who need less supervision while incarcerated and might be good candidates for early release.

“The tools are quite good at telling who’s not going to be violent,” Fazel told BuzzFeed News. But they’re not so great at identifying the rare individuals who pose the highest risk of committing acts of extreme violence.