Not guilty plea entered by a criminal defendant on the grounds that an impaired mental condition makes him or her unable to understand the wrongfulness of the act committed.
For most of U.S. history, insanity determinations were the province of the states, with intrastate consistency maintained by appellate court decisions and state statutes. Inconsistencies appeared among the states on a number of issues: the breadth of the insanity definition employed, permissible evidence and testimony, and the posttrial disposition of an insane defendant. Most states relied on the M’Naghten rule
In the mid-twentieth century, two contrasting cultural trends encouraged the Supreme Court to examine the procedures and consequences of the insanity defense. These trends were an emerging concern with the civil liberties of the mentally impaired criminal defendant and a growing fear of crime that highlighted the need of the state to protect the public from criminally insane individuals.
A series of Court decisions protected the due process
At the same time, the Court recognized the police powers
Public concern with insanity as a loophole grew most intense after 1983 when John Hinckley, attempted presidential assassin, was found not guilty by reason of insanity and hospitalized. Several states tried to reduce the use of insanity pleas by adding an alternative guilty but mentally ill category of offender, who was treated, then punished. Other state courts, in contrast, continued to employ definitions of insanity broader than that set forth in the M’Naghten rule, encompassing “irresistible impulses” by lucid people. Challenges to such definitional variations are consistently dismissed by the Court. Although the scope of insanity definitions varies widely, the underlying relevance of criminal intent remains broadly accepted and constitutionally solid.
Steadman, Henry J., et al. Before and After Hinckley: Evaluating Insanity Defense Reform. New York: Guilford Press, 1993. Wrightsman, Lawrence, Michael Nietzel, and William Fortune. Psychology and the Legal System. Brooks/Cole, 1998.
Due process, procedural
Indigent criminal defendants