Insanity Defense

The insanity defense is an affirmative defense in criminal law by which a defendant argues that a mental disease or defect at the time of the offense should relieve them of criminal responsibility for the act, a question distinct from a defendant's competence to stand trial, which concerns mental state at the time of trial rather than at the time of the crime. Its most influential test is the M'Naghten Rules, formulated by the House of Lords in 1843 after Daniel M'Naghten's acquittal for killing Edward Drummond; American courts and legislatures adopted the M'Naghten test with little modification for more than a century. Later jurisdictions developed alternatives to it. The irresistible impulse test, adopted by a number of American states, excuses a defendant who knew an act was wrong but whose mental disease left them unable to control the impulse to commit it, addressing M'Naghten's narrow focus on cognitive knowledge of right and wrong. The Durham rule, adopted by the District of Columbia Circuit in Durham v. United States (1954), asked only whether the crime was the product of mental illness; its broad and imprecise standard drew sustained criticism, and the circuit abandoned it in United States v. Brawner (1972). The American Law Institute's Model Penal Code test combines a cognitive and a volitional branch, excusing a defendant who lacked substantial capacity either to appreciate the criminality of the conduct or to conform the conduct to the requirements of law, and became the majority American standard through the 1970s. John Hinckley Jr.'s acquittal by reason of insanity in 1982, for the attempted assassination of President Ronald Reagan, prompted Congress to narrow the defense sharply in federal courts: the Insanity Defense Reform Act of 1984 discarded the Model Penal Code's volitional branch, confined the defense to a severe mental disease or defect, and shifted the burden of proof onto the defense at a clear-and-convincing standard, producing a federal test closer to M'Naghten than to the standard it replaced. An eight-state empirical study found the defense is raised in fewer than one percent of felony cases and succeeds, when raised, in roughly a quarter of those, most often by defendants who already carried a diagnosed mental health condition before the offense; the defense's popular reputation as a common escape from punishment is not supported by the record. Because the legal test asks a clinical question in legal terms, criminal responsibility at the time of the offense is assessed by forensic psychiatrists and forensic psychologists through what the discipline calls a mental state opinion, addressing whether the defendant could understand the nature and wrongfulness of the act when it was committed; the American Academy of Psychiatry and the Law, the discipline's professional body, states that insanity is a legal rather than a medical term, so the clinician's opinion informs, but does not substitute for, the court's own finding.