Also known as the “right-wrong” test, the M’Naghten Rule is a legal standard used to determine whether a defendant can be excused from criminal liability by reason of insanity.

Defendants are presumed to be sane and have a sufficient degree of reason to be responsible for their crimes. To rebut this presumption one of the following is required:

To establish a defence on the ground of insanity, it must be clearly proved that at the time of committing the act the party accused was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing, or as not to know that what he was doing was wrong.¹

The M’Naghten Rule remains the most common insanity standard across U.S. state jurisdictions as of the time of this writing.

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1) In a 2020 pronouncement about the insanity defense by the United States Supreme Court, the dissent identified this as: “the most famous statement of the traditional insanity defense, that contained in M'Naghten's Case” Kahler v. Kansas, 140 S. Ct. 1021, 1038, 206 L. Ed. 2d 312 (2020)

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