What is the definition of insanity?

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].

There is no single standard for considering an insanity defense in American courts of law. Individual states typically operate under one of two tests.

The M'Naghten test, developed in the U.K. in the 1840s after the Scotsman was originally set free, concentrates on a defendant's cognitive ability to understand that his actions are wrong. But the American Law Institute's (ALI) insanity test -- developed in 1962 and laid out in the Model Penal Code -- also considers what experts call the "irresistible impulse": a defendant's inability to refrain from doing something he knows is wrong. Under this test, a person is criminally insane if he is unable to "appreciate the criminality of his conduct or to conform his conduct to the requirements of the law." For example, a murderer who takes steps to avoid getting caught -- using gloves to prevent fingerprints or disposing of the body where it's unlikely to be found -- shows he's aware of the gravity of his crime, but may still be "insane" under the ALI standard. [sources: PBS, FindLaw].

Almost 20 years after the ALI test was created, John Hinckley Jr. changed the landscape of insanity defense law when he shot President Ronald Reagan and wounded three other men, permanently paralyzing press secretary James Brady. Hinckley claimed the assassination attempt was intended to impress actor Jodie Foster. Following a trial, a jury found Hinckley not guilty on various criminal charges by reason of insanity [sources: Muskal, PBS].

The Insanity Defense Reform Act of 1984 was born out of the public outcry that followed. The law, which applies to federal criminal courts considering an insanity plea, moved away from the ALI test to something more akin to the M'Naghten standard. Specifically, the law limits the defense to a person who "was unable to appreciate the nature and quality or the wrongfulness of his acts," and explains that "[m]ental disease or defect does not otherwise constitute a defense [sources: U.S. Department of Justice, PBS].

Perhaps more significantly, other post-Hinckley legislation -- federal and state -- shifted the burden of proof in an insanity defense from the prosecution to the defendant. In the new regime, prosecutors no longer need prove beyond a reasonable doubt that a defendant was sane at the time of the crime. Rather, the defendant must provide "clear and convincing evidence" that he was legally insane [sources: US Legal, PBS].

To prove insanity, the defense must establish that a mental illness prevented the defendant from understanding that his actions were wrong at the time of the offense.

And they must do it with "clear and convincing evidence." In federal courts, jurors are instructed that the term refers to evidence that makes it "highly probable" that the person was insane when she committed the crime. Jurors are permitted to consider the defendant's mental condition before and after the crime, as well as witness testimony and the opinions of the psychiatric experts, which they may accept or reject as they see fit, in reaching this decision [source: Legal Information Institute].

Regardless of the specific terms used to describe the evidence, the defense must establish a clear connection between the defendant's mental illness and the actual commission of a crime. Just because a person suffers from episodes of psychosis, doesn't mean that the psychosis caused her to rob a liquor store. Maybe she was just thirsty [source: Singer-Vine].

The insanity question often comes down to expert opinion. Mental health professionals typically interview a defendant and people who know her -- family, friends, co-workers -- to determine insanity. They also review other information like the arrest warrant and charging documents, the person's medical and criminal history and any statements made on social media sites like Facebook and Twitter. In addition, experts rely on specialized tests designed to catch defendants embellishing their impairments by claiming that they experience exaggerated symptoms or those that are not typically associated with a specific disorder [source: Singer-Vine].

The results of these reviews often weigh against the accused. A 2004 study of more than 5,000 sanity evaluations conducted by forensic evaluators in Virginia showed that defendants were deemed insane in just 15 percent of the cases [sources: Warren, Singer-Vine].

If all the legal jargon, rules and statistics haven't driven you to the brink of insanity, read on for more information about mental health and criminal justice.