Mental-health experts are wrong. Untreated, the seriously mentally ill are prone to violence. While virtually the entire nation unites around the reasonable proposition that people with serious mental illnesses should not own assault weapons, one group takes umbrage: mental-health experts. In the wake of incidents such as the one at Newtown, the experts immediately issue press releases claiming that people with mental illness are no more violent than others, leading to the conclusion that people with serious mental illness should not be the target of gun-control efforts.How can the chasm be so wide? Who is right? The public that believes...