The Washington Posts says progress has been made in the decades-long effort to try to figure out whether there’s a link between mental illness and violence, and if so, to predict which people are likely to act. It’s now fairly clear, for example, that people with severe mental illness 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. “There is no instrument that is specifically useful or validated for identifying potential school shooters or mass murderers,” said one expert. Even when someone has a history of threatening behavior, the killing of innocent people can’t necessarily be prevented. 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.