One of the most contentious issues surrounding mental illness is the debate over how the legal system should deal with the mentally ill who commit crimes. The insanity defense was adopted so that mentally ill criminals could receive psychiatric treatment instead of being sent to jail. However, debate flourishes over whether the defense is a loophole for criminals or a way to provide help to defendants who cannot distinguish between right and wrong.

Even trickier is the question of how to determine the criminal responsibility of defendants who suffer from multiple personality disorder (MPD)—a disorder in which a person demonstrates one or more “alter” personalities. In one such case, Thomas Huskey was accused of murdering four women in Knox County, Tennessee. While Huskey declares to have no memory of committing the crimes, his...