Each state, and the District of Columbia, has its own statute outlining the standard for determining whether a defendant is legally insane, therefore not responsible, at the time the crime is committed.
“An insanity defense is based on the theory that most people can choose to follow the law; but a few select persons cannot be held accountable because mental disease or disability deprives them of the ability to make a rational / voluntary choice. Such individuals need special treatment as opposed to prison; punishment is not likely to deter future antisocial conduct of these mentally diseased individuals.” Retrieved on 5/25/2010 from http://www.law.umkc.edu/faculty/projects/ftrials/hinckley/EVOL.HTM
There are basically two categories, the M’Naghten Rule and the American Law Institute Model. The states are pretty much split between these two categories, with the exception of Montana, Idaho, and Utah which do not allow for an insanity defense.
The M'Naghten Rule provides as follows:
"Every man is to be presumed to be sane, and ... that to establish a defense on the ground of insanity, it must be clearly proved that, at the time of the committing of the act, the party accused was laboring under such a defect of reason, from disease of mind, and not to know the nature and quality of the act he was doing; or if he did know it, that he did not know he was doing what was wrong."
Retrieved on 5/21/10 from http://criminal.findlaw.com/crimes/more-criminal-topics/insanity-defense/the-mnaghten-rule.html
The test used under the M’Naghten standard, commonly referred to as the "right/wrong" test, to determine if a defendant can distinguish right from wrong is based on the idea that the defendant must know the difference of each in order to be convicted of the crime. Some states that use the M'Naughten rule have modified it to incorporate a provision for a defendant suffering under "an irresistible impulse" that would prevent him from being able to stop himself from committing an act he knows is wrong.
After advances in the world of psychiatry, new interests in the field of psychology, new medications thought to cure mental disease, and much frustration with the M’Naghten Rule, the courts adopted a new standard – The Durham Test. The test provided that:
“A person was not criminally responsible if the unlawful act was a product of mental disease or defect. A jury was required to answer two questions: (1) did the defendant have a mental disease or defect? and (2) if so, was the disease or defect the reason for the unlawful act? Both of the answers had to be "yes" to return a verdict of not guilty by reason of insanity. The test was a recognition of that mental illness was a disease that could be treated and possibly cured.” Retrieved on 5/25/2010 from http://www.law.umkc.edu/faculty/projects/ftrials/hinckley/EVOL.HTM
In 1962, due to much criticism of the various tests for the insanity defense, the American Law Institute (ALI)...