In response to the criticisms of the various tests for the insanity defense, the American Law Institute (ALI) designed a new test for its Model Penal Code in 1962. Under this test, “a person is not responsible for criminal conduct if at the time of such conduct as a result of mental disease or defect he lacks substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of the law.”
The Model Penal Code test is much broader than the M’Naghten Rule and the Irresistible Impulse Test. It asks whether defendants have a substantial incapacity to appreciate the criminality of their conduct or to conform their conduct to the law rather than the absolute knowledge required by M’Naghten and the absolute inability to control conduct required by the Irresistible Impulse Test.
The ALI test also requires that the mental disease or defect be a mental diagnosis. In this way, it manages to incorporate elements of all three of its predecessors: the knowledge of right and wrong required by M’Naghten, the prerequisite of lack of control in the Irresistible Impulse Test, and the diagnosis of mental disease and defect required by Durham.
Such a broad based rule received wide acceptance, and by 1982 all federal courts and a majority of state courts had adopted the ALI test. While some states have since dropped the ALI test, and it no longer applies at the federal level, 18 states still use the ALI test in their definitions of insanity.