(ALI) produced another test for the insanity defense in its Model Penal Code (MPC). The first part of this test said that a person is not responsible for criminal conduct if "as a result of mental disease or defect he lacks the substantial capacity either to appreciate the criminality [or wrongfulness] of his conduct or to conform his conduct to the requirements of the law." The second paragraph of this test excluded "an abnormality manifested only by repeated criminal or otherwise anti-social conduct" from the definition of "mental disease or defect." What this test did, in effect, was to combine the M'Naghten test with the "irresistible impulse" test (requiring a finding of not guilty by reason of insanity if the defendant had a mental disease which kept him from controlling his conduct). However, the ALI test only required a lack of "substantial capacity," in contrast to the M'Naghten rule, which required a complete impairment of cognitive capacity (and capacity for self-control, under the irresistible impulse test).
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