Holmes Wants to Plead Insanity For Colorado Theater Shooting. What Does Insanity Mean?

Lawyers for James Holmes, the accused Colorado theater shooter, are requesting that Holmes' plea be changed to not guilty by reason of insanity. This is the kind of thing that makes people crazy. It can seem like a lawyerly trick to help the guilty go free. So I thought it might be useful to outline how a psychologist understands Colorado lawyers claimimg someone is not responsible for a crime because they are legally insane.