We've all heard of the "insanity" defense from the movies. It might go something like this: A man is accused of murder, but he is able to show in court that -- although he committed a killing -- the incident cannot be considered murder because the defendant was out of his mind when the killing happened. The defense of insanity against murder charges is, in fact, a legitimate and viable defense in some murder cases, but where did it come from?

The first recorded reference to the "insanity defense" dates back to 1581, when a legal treatise stated, "If a madman or a natural fool, or a lunatic in the time of his lunacy" were to kill another person, this individual will not be held criminally accountable. In order to test whether someone was actually insane or out of his or her mind, old English courts later created the "wild beast" test in the 1700s. This test said that defendants would not suffer conviction of murder in cases where the defense proved that their client did not understand the nature of the crime better than "an infant, a brute, or a wild beast."

The terms "brute," "wild beast" and "lunatic" are no longer used by modern courts to describe defendants accused of crimes. However, the general idea behind this old English doctrine remains in effect in varying ways. Depending on the court deciding the matter and the facts of the case, it's possible that:

• Defendants won't be found guilty of murder when they didn't understand what they did, failed to distinguish between right and wrong or suffered from a "disease of mind."
• Defendants won't be found guilty of murder if they were shown to be unable to control their impulses leading to criminal actions.
• Defendants won't be found guilty of murder if their mental defects resulted in acting in a criminal way.
• Defendants won't be found guilty of murder if they have a diagnosed mental defect.

Every case is different, but when the "insanity defense" applies to a certain factual scenario, it can be a powerful defense tool. That said, there might not be numerous situations in which this legal standard will apply.

Source: Findlaw, "Insanity Defense," accessed March 15, 2018