Pleading Insanity in a Criminal Case

A successful insanity defense usually results in many years of mandatory treatment in a mental hospital, not a free ride out of jail.

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Gavel and Scales

When defendants plead not guilty by reason of insanity, they are asserting an affirmative defense—that is, they admit that they committed a criminal act, but seek to excuse their behavior by reason of mental illness that satisfies the definition of legal insanity. People who are adjudged to have been insane at the time they committed a crime are neither legally nor morally guilty.

For more information on affirmative defenses in general, see Affirmative Defenses in Criminal Cases.

Historical Antecedents

The insanity defense has been around for centuries. A 1313 English court referred to insane people as “the witless, who do not have reason whereby they can choose the good from the evil.” More colorfully, an 1812 English court decided that a man who had shot a Lord was insane because he was “a madman who … doth not know what he is doing, no more than a brute or a wild beast.” Despite this lengthy pedigree, consensus on the proper definition of legal insanity still does not exist either among psychiatrists or among legal scholars, and the two professions don’t have a lot of confidence in each other.

Definitions of Legal Insanity

Many criminal acts seemingly result from distorted mental processes. The criminal justice system continues to struggle for a method to distinguish offenders whose mental illness is so severe that society should deem them not morally responsible for their behavior, from offenders whose actions, while perhaps objectively irrational, nevertheless merit punishment.

The M’Naghten test

Many states define legal insanity according to the M’Naghten Test, developed in an 1843 English case. An offender is insane under this test if mental illness prevents the offender from knowing the difference between right and wrong. Other states have replaced the M’Naghten Test with a modified version known as the Brawner Test. Under this test, defendants are insane if, because of mental disease or defect, they lack the substantial capacity to appreciate the criminality of their actions or to conform their behavior to legal requirements.

The “irresistible impulse” test

Some states supplement the M’Naughten or Brawner test with the irresistible impulse rule, under which offenders are insane if a mental disorder prevents them from resisting the commission of an illegal act that they know is wrong.

EXAMPLE: Manion, an army officer, returns home just as Quill races out the back door. Manion hurries inside and finds his wife Laura lying on the floor, raped and beaten by Quill. Manion picks up a gun, walks to Quill’s place of employment, shoots and kills him, then calls the police. A defense psychiatrist testifies that Laura’s injuries caused Manion to suffer a sudden psychic shock called dissociative reaction, and that dissociative reaction creates an unbearable tension that people may try to alleviate by taking immediate and often violent action. The psychiatrist’s testimony supports a conclusion that Manion was legally insane under the irresistible impulse test. (This example is loosely based on the classic 1959 film, Anatomy of a Murder.)

Trial Procedures

Defendants have to advise prosecutors prior to trial if they plan to rely on an insanity defense. Typically, defense lawyers and prosecutors each obtain their own psychiatrists to examine a defendant and testify at trial. Judges appoint government-paid psychiatrists for indigent defendants. Defendants have the burden of convincing judges or juries by either a preponderance of the evidence or by the tougher standard of clear and convincing evidence that they were insane at the time they committed a criminal act. Evidence rules forbid defense psychiatrists from testifying to an opinion that a defendant was legally insane at the time a crime was committed. They can only provide a medical diagnosis concerning a defendant’s mental illness.

Not a “Get Out of Jail Free” card

Defendants found not guilty by reason of insanity are rarely set free. Instead, they are almost always confined in mental health institutions. They may remain confined for a longer period of time than had they been found guilty and sentenced to a term in prison. States may compel defendants adjudged insane to remain in a mental health institution until they convince a judge that they are no longer legally insane.

Myths and Truths

Research has dispelled many popular myths suggesting that the insanity defense is a boondoggle that lets criminals “get away with it” and get back on the streets immediately. Research consistently produces the following conclusions:

  • Defendants offer an insanity defense in less than 1% of all felony cases, and are successful only about one-quarter of the time.
  • Defendants found not guilty by reason of insanity are often confined in mental institutions for many years, and in some cases for a longer time than they would have been incarcerated had they been found guilty.
  • Few offenders “fake” insanity; most defendants who plead insanity have a long history of mental illness and prior hospitalizations.
  • In the large majority of cases, prosecution and defense expert psychiatrists agree on whether defendants are legally insane.

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