Insanity itself is not recognized as a medical condition, but there are a variety of disorders that could cause a person to become legally insane.
Daniel M'Naghten, the Scottish woodcutter for whom an insanity test still used in both British and American jurisdictions is named, may have suffered from a psychotic disorder, such as paranoid schizophrenia. This chronic mental illness is characterized by a form of psychosis that causes a person to lose touch with reality, at times to the extent of hallucination and delusion. M'Naughten shot and killed a man in an 1843 attempt to assassinate Sir Robert Peel, the British prime minister whom M'Naghten believed had directly orchestrated his various personal and financial failings. M'Naghten was originally found not guilty by reason of insanity, but the verdict was later overturned [source: PBS].
Nevertheless, a defendant suffering from schizophrenia or other psychotic disorders is shown to have a better chance at an insanity defense than a person with a personality disorder, such as obsessive-compulsive disorder [source: Singer-Vine].
Mood disorders -- major and postpartum depression and bipolar disorders, for example -- may also qualify for an insanity defense. These ailments are more than just a bout with the blues. Among other symptoms, they can manifest themselves in severe irritation and agitation, slowed thinking and recurrent thoughts of death [source: Mayo Clinic].
In 2001, Andrea Yates killed her five children by drowning them one by one in the bathtub at her family's Houston home. She was initially convicted of murder, but later acquitted by reason of insanity. Yates had a history of mental illness, including postpartum depression, and had previously attempted suicide [source: Moisse].
The Yates case, however, is an outlier. Getting inside a person's head isn't easy. Neither is convincing a judge or jury of a criminal defendant's insanity. The defense is used in just roughly 1 percent of U.S. criminal prosecutions and is successful in only about a quarter of those cases [sources: Lally, Madsen].