It’s now fairly clear, for example, that people with severe mental illness, such as schizophrenia, bipolar disorder and some personality disorders, are more likely to commit violent acts than others. But the risk is small. The vast majority of mentally ill people won’t commit assault, rape, arson or homicide, although the risk rises sharply among those who abuse drugs and alcohol.
These insights are proving useful to psychiatrists, psychologists, judges, school administrators and others who must decide whether someone seems too dangerous to be left alone. But they aren’t good enough to identify an Adam Lanza, the young man who killed 28 people, including himself, in Newtown, Conn., last month. (Lanza’s mother told friends that he had Asperger syndrome, a developmental disorder, but no evidence has emerged that Lanza was diagnosed as mentally ill.)
“There is no instrument that is specifically useful or validated for identifying potential school shooters or mass murderers,” said Stephen D. Hart, a psychologist at Simon Fraser University in Vancouver who is the co-author of a widely used evaluation tool. “There are many things in life where we have an inadequate evidence base, and this is one of them.”
Even when someone has a history of threatening behavior, the killing of innocent people can’t necessarily be prevented.
The woman accused of pushing a man to his death in front of a New York subway train on Dec. 27 had been arrested several times for assault and treated in the psychiatric wards of two hospitals. The man who fatally shot two firefighters and himself in Webster, N.Y., on Christmas Eve had killed his 92-year-old grandmother three decades earlier.
The task of identifying violence-prone individuals is even trickier with young people, who have shorter histories and whose normal development often includes a period of antisocial behavior.
The prospect that the most recent massacre, or the next one, could lead to efforts to find young men contemplating the killing of strangers worries many people. Among those expressing concern are some psychologists and former patients forcibly swept into the mental health system and treated against their will.
“I think people are going toward wanting all their kids to be screened in high school for mental illness and violence risk — and that’s a bad idea,” said Gina M. Vincent, a forensic psychologist at the University of Massachusetts Medical School. “That’s my biggest fear of what’s going to come out of this.”
“We can’t go out and lock up all the socially awkward young men in the world,” said Jeffrey W. Swanson, a professor of psychiatry and behavioral sciences at Duke University. “But we have to try to prevent the unpredicted.”