The insanity defense is one that is described as a legal defense based on claims of mental illness or mental incapacity. This defense has received much attention from the media, including movies and television shows that highlight this defense as a common practice. In reality, the insanity defense is rarely used.
An important thing to note here is that, in the legal realm, insanity does not have a medical definition; only a legal definition. The legal definitions of insanity have very little to do with the psychological or psychiatric aspects of mental illness. Legal insanity is a concept that was developed in assigning guilt or innocence to particular defendants, and is not meant to understand the origins of mental pathology or its treatments.
Whenever an insanity defense is mentioned, you will almost always hear the M"Naughten Rule or M"Naughten Test mentioned as well. This is a rule (or test) for determining insanity. According to Frank Schmalleger's "Criminal Law Today," this rule holds that a person is "not guilty of a crime if, at the time of the crime, the person either didn't know what he or she was doing or didn't know that what he or she was doing was wrong." .
According to the M"Naughten rule, the inability to distinguish right from wrong must be the result of some mental defect or disability. Although this rule is still followed in many states today, in most states, the burden of proving insanity falls upon the defendant. Just as defendants are considered innocent until proven otherwise, they are also considered to be sane at the trial unless otherwise proven.
Another rule for gauging insanity is called the Durham rule. This rule states that a person is not criminally responsible for his/her behavior if that person's actions were the caused by some mental disease or defect. Using the Durham rule, courts hear from an array of psychiatric specialists as to the mental state of the defendant.