insanity

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Insanity is a mental illness or disease that prevents a person from fully understanding their actions. While insanity is primarily a criminal law concept, it can also be found in the laws of contracts and wills. Insanity can be either partial or complete and temporary or permanent. Definitions of insanity are often found in statutes, especially state criminal codes, but can also originate from judicial decisions.

Criminal insanity is insanity that prevents a defendant from knowing or understanding the nature of their actions or distinguishing right from wrong. The definition of insanity varies by state, with most states following either the M’Naghten rule or the Model Penal Code (MPC) insanity defense. Other tests for criminal insanity include the irresistible impulse test and the Durham test.

Based on the M’Naghten rule, the California Penal Code §25 defines insanity as the defendant being “incapable of knowing or understanding the nature and quality of his or her act and of distinguishing right from wrong at the time of the commission of the offense.” In contrast, the District of Columbia Circuit Court adopted the MPC definition of insanity in US v. Brawner (1972), holding that “A person is not responsible for criminal conduct if at the time of such conduct as a result of mental disease or defect he lacks substantial capacity either to appreciate the criminality [wrongfulness] of his conduct or to conform his conduct to the requirements of the law.” The MPC’s requirement of capacity to “appreciate” is considered broader than the M’Naghten rule’s requirement of “knowing or understanding.” 

A defendant can raise an insanity defense to negate the culpability required for conviction of the relevant crime. A successful defense will relieve the defendant of criminal responsibility and usually lead to confinement in a state hospital for psychiatric treatment. 

For contracts, insanity can render a person incapable of executing a valid contract when they lack the capacity necessary for a full understanding of the nature and consequences of the contract. Contracts entered by a person deemed insane will be considered void. Some states do not separate insanity from incompetence for purposes of contract law. 

[Last updated in June of 2023 by the Wex Definitions Team]