m'naghten test n
[after Daniel M'Naghten, defendant in 1843 murder case heard before the British House of Lords who was acquitted due to his insanity]
: a standard under which a criminal defendant is considered to have been insane at the time of an act (as a killing) if he or she did not know right from wrong or did not understand the moral nature of the act because of a mental disease or defect called also M'Naghten rule; compare diminished capacity, durham rule, irresistible impulse test, substantial capacity test NOTE: Many jurisdictions have followed the Model Penal Code in basing criminal insanity on either of two factors: an inability to appreciate the wrongfulness of an act, which reflects the influence of the M'Naghten test, and an inability to conform one's behavior to the dictates of the law, which reflects the concept of the irresistible impulse. Both factors must be rooted in a mental disease or defect, which is also what the Durham rule requires of insanity.
Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.