The Durham Rule - Irresistible Impulse

The M'Naghten Rule - Not knowing Right From Wrong

The history of "not guilty by reason of insanity"

The insanity defense reflects a compromise on the part of society and the law. On the one hand, society believes that criminals should be punished for their crimes; on the other hand, society believs that people who are ill should receive treatment for their illness. The insanity defense is the compromise: basically, it reflects society's belief that the law should not punish defendants who are mentally incapable of controlling their conduct.

In the 18th century, the legal standards for the insanity defense were varied. Some courts looked to whether the defendant could distinguish between good and evil, while others asked whether the defendant "did not know what he did." By the 19th century, it was generally accepted that insanity was a question of fact, which was left to the jury to decide.