M'Naghten's Case, (1843) 10 Cl & Fin 200, 8 Eng. Rep. 718 (H.L.)
M'Naghten's Case is the canonical common-law source for the modern insanity defense. Decided in 1843 amidst intense public scrutiny, the case produced the "M'Naghten Rules," which define when a mental disorder negates criminal responsibility.
What legal standard governs when a criminal defendant should be excused from liability on the ground of insanity?
The M'Naghten Rules (as articulated by the judges and adopted by the House of Lords): 1) Presumption of sanity: Every person is presumed sane and to possess sufficient reason to be responsible for crimes until the contrary is clearly proved. 2) Cognitive defect from disease: To establish the defense of insanity, it must be clearly proved that, at the time of the act, the accused was laboring under such a defect of reason, from disease of the mind, as either (a) not to know the nature and quality of the act he was doing; or (b) if he did know it, that he did not know that what he was doing was wrong. 3) Delusions: Where the accused labors under a partial delusion, he must be considered as if the facts believed to exist were real; criminal responsibility then turns on whether, on those supposed facts, the act would have been justified or excused. 4) Expert testimony: Medical evidence is admissible to show the existence of disease of the mind and its effect on the defendant's reasoning, but the ultimate question of responsibility is for the jury under the legal standard. 5) Volitional control: An "irresistible impulse" or mere failure of self-control, without the required cognitive defect, does not suffice to excuse criminal conduct.
In response to the House of Lords' certified questions, the common law judges articulated and the House adopted the M'Naghten Rules: a defendant is excused only if, due to a disease of the mind causing a defect of reason, he did not know the nature and quality of his act or did not know that it was wrong; sanity is presumed, delusions are treated as if true for legal analysis, and lack of volitional control alone is not a defense.
M'Naghten is the foundational common-law authority for the insanity defense's cognitive test. Its rule (or close variants) remains the dominant standard across many U.S. jurisdictions and continues to inform English law, where later cases have elaborated concepts like "disease of the mind" and the meaning of "wrong" (often understood as legal wrongfulness). The case shaped the allocation of proof (presumption of sanity; defendant's burden to establish insanity) and famously excludes a pure "irresistible impulse" defense absent cognitive incapacity. It also spurred later reforms and critiques: some jurisdictions supplemented M'Naghten with a volitional prong, others replaced it (e.g., the Model Penal Code's "lack of substantial capacity" test), and Congress narrowed the federal defense after the Hinckley verdict. For law students, M'Naghten is indispensable: it frames exam analysis of insanity, anchors comparative study of doctrinal alternatives, and illustrates how legal standards mediate between psychiatric science and normative judgments about criminal blame.