The tendency of the United States criminal justice system has been to limit the degree to which one can claim innocence based on mental illness. During the middle of the 20th century many courts, through the Durham Rules and the American Law Institute Model Penal Code, regarded impaired volition as legitimate grounds for the insanity defense. However, when John Hinckley was acquitted due to insanity in 1982, a reversal of this opinion occurred, which spurred a narrowing definition of mental illness. Insanity decisions became increasingly based on the M’Naghten Rules, which asserted that unless one was able to prove that a mental illness kept him or her from knowing that their actions were wrong, or knowing the disposition of the criminal act, one would not be able to be tried as criminally insane.