The defense is based on the principle that punishment is only reasonable if the defendant is capable of controlling their behavior and understanding that they have commited a wrongful act. Because some people suffering from a mental disorder are not capable of knowing or choosing right from wrong, they should not be criminally punished.
The concept has existed since ancient Greece and Rome, being re-codifed across the centuries, the first full transcipt for an insanity trial dates from 1724. But despite the antiquity of the concept the insanity defense is still open to controversy and defendants rarely enter such pleas. The problems centre around the definition of insanity in the criminal law context. Legal definitions are repeatedly refined through cases - but a popular definition is the McNaghten rule which defines insanity as "the inability to distinguish right from wrong." It dates from England, 1844 (based on a 1843 trial) and was first applied in the US in 1865. Another is he irresistible impulse test - a person may know that an act is wrong, but because of mental illness cannot control their actions. The Durham test (1955) staes that "An accused is not criminally responsible if his unlawful act was the product of mental disease or mental defect".
Defendants found not guilty by reason of insanity are only rarely freed. They are usually confined to a mental institution and defendants can often spend more time there they would have in prison had they been convicted.
An insanity defense rests on the testimony of a psychiatrist?, who testifies for the defendant after examining him and his past history, and the facts of the case.
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