Not mentally responsible, to some degree (the degree depending on the legal transaction in relation to which it is employed). 232 F. Supp. 255, 257. The term may be used to signify lack of criminal responsibility, need for commitment to a mental institution, inability to transact business, inability to stand trial (i.e., unable to assist in one's own defense). See 214 A. 2d 393, 405. "In criminal law, ‘insanity,' by whatever test it may be ascertained, may be said to be that degree or quantity of mental disorder which relieves one of the criminal responsibility for his actions." 316 P. 2d 917, 919. It is a legal, not medical, standard. Compare incompetence. See also diminished capacity; non compos mentis; not guilty [not guilty by reason of insanity].
There are three main tests governing insanity defenses:
The m'naghten rule was the common law test of criminal responsibility. 8 Eng. Rep. 718. A person was not responsible for criminal acts if as a result of a mental disease or defect he or she did not understand what he or she did or that it was wrong, or was under a delusion (but not otherwise insane) which, if true, would have provided a good defense. The person is unable to distinguish right from wrong. See M'Naghten rule.
The first major modification of the M'Naghten Rule was the durham rule which states that "an accused is not criminally responsible if his unlawful act was the product of mental disease or defect." 214 F. 2d 862, 874-75. See Durham rule.
Some jurisdictions employ an irresistible impulse test, sometimes referred to as an uncontrollable impulse test or emotional insanity. It applies when a defendant understands the nature and consequences of his or her act and understands that it is wrong but is unable to resist the impulse to commit the act because of mental disease or defect.
Most federal courts and many state courts and legislatures have adopted the American Law Institute's Model Penal Code §4.01(1) (1962) test which combines many of the elements of the M'Naghten and Durham tests. The ali test (which is synonymous with the model penal code test or the substantial capacity test) states that "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 [or alternatively, wrongfulness] of his conduct or to conform his conduct to the requirements of law." The ali test further provides that the "terms ‘mental disease or defect' do not include an abnormality manifested only by repeated criminal or otherwise anti-social conduct." Id. At §4.01(2). This language is designed to exclude the sociopathic or psychopathic criminal.
Since a person found to be insane at the time of the commission of the criminal act could not have the necessary mental state to commit a crime, the finding of insanity is a complete acquittal to the charge. Moreover, since such a finding is based on the defendant's state at the time of the crime, the finding does not necessarily relate to the defendant's mental state at the time of trial and therefore does not logically lead to the conclusion that the defendant is sufficiently incompetent to be involuntarily committed in a mental institution.
Further, insanity provides a ground for rescinding a contract or will when it is shown that the contracting party did not understand the nature of his or her act or the extent of his or her property.
The insanity defense must be distinguished from the concept of "capacity to stand trial," which is based on the defendant's ability to assist in his defense and his understanding of the charges against him. See Beran & Toomey, Mentally Ill Offenders and the Criminal Justice System 11 (1979). While the insanity defense affects a person's culpability, lack of capacity to stand trial does not bar a subsequent trial for the charges against him when and if the condition ceases. See capacity, competent.