The defense of insanity asserts that, at the time of the alleged criminal act, that the defendant lacked the mental responsibility sufficient for the mens rea of the crime. At common law, if the defendant was legally insane at the time of the criminal act, no criminal liability will be imposed on them.
Jurisdictions use four different tests to define legal insanity: (1) the M'Naghten test, (2) the Irresistible Impulse test, (3) the Model Penal Code, and (4) the Durham Rule.
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