A criminal defendant accused of a crime can argue that they are not responsible because they have a mental illness. In this case, the defendant would plead "not guilty by reason of insanity." Pleading "not guilty by reason of insanity" is not to be confused with "diminished capacity."

While both have to do with mental incompetence, there are differences. Most importantly, "by reason of insanity" is a full defense to a crime – it's equivalent to pleading "not guilty," while "diminished capacity" means to plead to a lesser crime.

Society believes that criminals should be punished for their crimes, however, it also believes that sick people should receive treatment when they are mentally ill.

With the insanity defense, we have come to a compromise: basically, society contends that the criminal justice system shouldn't punish defendants who are mentally incapable of controlling their actions and behavior.

Court Tests for Legal Insanity

Each jurisdiction utilizes different tests for legal insanity:

• "M'Naghten Rule" – Due to a "disease of the mind," the defendant failed to distinguish right from wrong, or they didn't understand what they did wrong.
• "Durham Rule" – Defendant's mental defect was the cause of the criminal act.
• "Model Penal Code" Test for Legal Insanity – Due to the diagnosed mental defect, the defendant failed to understand the criminal nature of their acts, or was unable to abide by the law.
• "Irresistible Impulse" Test – Because of the defendant's mental disease, they were unable to control their impulses, which were responsible for the criminal behavior.

When a defendant is found guilty by reason of insanity, he or she is usually required to receive psychiatric treatment; such defendants are usually placed in a mental institution.

Unlike other defendants, they are not institutionalized for a set period of time, but rather held indefinitely until they are no longer perceived as a threat to society. Since authorities don't take institutionalization lightly, defendants are often institutionalized longer than they would be if they were sent to a prison.