Insanity defense

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A plea entered during a criminal trial that the person charged with the crimes should not be punished on the grounds that they were not of sound mind at the time of he offence. The actual plea is "not guilty by reason of insanity." (NGRI). This defense is very rarely invoked and even more rarely successful. However because trials in which this defense is invoked tend to be dramatic with much media coverage, the public believes that this defense is used and abused far more often than is the case.

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 rules which define 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 the 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". After the 1970's, jurisdictions within the United States have moved tended to away from the Durham test partly because this test puts too much emphasis on the opinions of psychiatric experts.

In common law jurisdictions, an insanity defense rests on the testimony of psychiatrists who examine the defendant after examining him and his past history. In cases where the insanity defense is invoked, both the prosecution and the defense will retain independent psychiatrists who will each examine the defendant. In cases where the teams of psychiatrists agree, the prosecution and the defense will come to an agreement about the outcome of the case and the case will not go before a jury. In cases where the teams of psychiatrists disagree, both sides will present their cases before a jury who will decide who to believe.

Defendants found not guilty by reason of insanity are generally placed in a mental institution. Unlikely defendants who are found guilty of a crime, they are not institutionalized for a fixed period, but rather they are held in the institution until medical and legal authorities determine that they are no longer a danger to themselves or others.

This is a difficult determination to make, and because there is an extreme public reaction if someone is incorrectly released while there is little public reaction if someone who is safe is held in the institution, authorities making this decision tend to be extremely cautious. As a result, defendants are can often spend more time there they would have in prison had they been convicted.