The Insanity Defense in Criminal Law Cases

The Criminal Defense of Insanity

Defendants who are determined to have been insane at the time they committed a crime are entitled to the criminal defense of not guilty by reason of insanity. This defense has been controversially applied over the years, for it has resulted in not guilty verdicts in several high-profile cases. As a result, there is often a general public sentiment that the insanity defense is too frequently applied to criminal defendants. In reality, however, various criminal studies have established that only about one percent of all felony cases in the United States involve use of the insanity defense. Moreover, even when the defense is asserted, it is successful in only about 30 cases every year.

History of the Insanity Defense and the Burden of Proof

For several hundred years, the insanity defense has acted as a defense against criminal charges for a defendant who was incapable of understanding what he or she was doing, or in determining right from wrong. Since our criminal system generally requires that most defendants had some knowledge or intent when committing a criminal act, the insanity defense provides relief for those deemed incapable of forming such mental states.

Originally, most states required that, when a defendant asserted a defense of insanity, the prosecutor was required to prove beyond a reasonable doubt that the defendant was not insane. However, in 1982, John W. Hinckley was acquitted of the attempted assassination of President Ronald Reagan on the basis of an insanity defense, and this result caused many states to reform their insanity laws. Many states shifted the burden of proof from the prosecutor to the defense, requiring defense attorneys to show by clear and convincing evidence or by a preponderance of the evidence that the defendant was insane. In Idaho, Kansas, Montana, and Utah, the defense of insanity was eliminated entirely. Instead, defendants may be found “guilty, but insane” and sentenced to psychiatric institutionalization instead of prison.

For federal crimes, the Hinckley case led to the passage of the Insanity Defense Reform Act, which requires that defendants prove the defense of insanity by clear and convincing evidence.

Establishing Insanity

Currently, states rely on four different tests to determine whether a defendant is legally insane. The laws of your state will establish which of these four tests applies.

Under the M’Naghten Rule test, the criminal defendant must either not understand what he or she did, or be unable to distinguish right from wrong. This test relies on the notion that defendants may be diseased such that they are incapable of understanding their own actions.

The Durham Rule holds that if a criminal defendant’s “mental disease or defect” was the reason that he or she committed a crime, the defendant is not guilty by reason of insanity. This test is currently used only in New Hampshire, since it has been deemed too broad by other states and jurisdictions.

The irresistible impulse test is used to determine whether, as a result of a mental disease or defect, a defendant was unable to control or resist his or her own impulses, thus leading to a criminal act. If so, the defendant is not guilty by reason of insanity. Some states consider this test in combination with the M’Naghten test, holding that when a defendant could not understand his or her own actions, nor control them, the insanity defense applies.

Under the Model Penal Code, the insanity defense applies when, because of a diagnosed mental disorder, the defendant could not understand the criminality of his or her actions or was unable to “act within the confines of the law.” This test is still used in many states, but it was criticized after it led to the acquittal of John Hinckley.

Last reviewed October 2024