M'Naghten Rule

The M'Naghten rule is the oldest and most widely used test for legal insanity in the United States. It originated from the 1843 English case of Daniel M'Naghten, who killed the secretary to the Prime Minister while suffering from paranoid delusions. The rule focuses exclusively on cognitive impairment — the defendant's ability to understand what they were doing.

Under the M'Naghten test, a defendant is legally insane if, at the time of the act, they were suffering from a defect of reason caused by a disease of the mind, and as a result either (1) did not know the nature and quality of the act they were performing, or (2) did not know that what they were doing was wrong. The test has two prongs, and satisfying either one establishes insanity.

The first prong (not knowing the nature and quality of the act) covers situations where the defendant is so mentally impaired that they do not understand the physical nature of their conduct — for example, a person who strangles someone believing they are squeezing a lemon. The second prong (not knowing the act was wrong) is more commonly invoked and covers situations where the defendant knows what they are doing but does not appreciate that it is wrong.

Jurisdictions differ on whether "wrong" means legally wrong (against the law) or morally wrong. Some courts hold that a defendant who knows their act is illegal but believes it is morally justified is not insane under M'Naghten. Others define "wrong" more broadly to include moral wrongness.

The M'Naghten rule is criticized as too narrow because it focuses only on cognitive understanding and ignores volitional impairment — a person who knows their act is wrong but cannot control their behavior due to mental illness is not excused under M'Naghten. This criticism led to the development of broader tests, including the irresistible impulse test and the Model Penal Code test.

On criminal law exams, students should identify which insanity test the jurisdiction uses and apply it carefully to the facts. M'Naghten requires a clear showing of cognitive impairment — mere mental illness is insufficient without proof that it affected the defendant's knowledge.