What does not guilty by reason of insanity mean?

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In movies and on television shows, a standard legal defense for a criminal defendant is insanity or temporary insanity. We also hear about this from time to time in real life, of course, but it is not an especially common legal defense. To most of us, the legal reasoning behind this defense is fairly mysterious even though we've probably seen it played out dozens of times. On a typical lawyer show, the defense lawyer brings in a psychologist that says that the defendant should not be held accountable for his or her actions because he or she has a certain mental illness that interferes with his or her reasoning capacity. If the jury thinks the person actually does have this mental illness, it finds him or her not guilty.

This raises a number of questions that most shows don't answer very clearly:

  • Why does being mentally ill excuse somebody from criminal guilt?
  • How is a jury of regular people qualified to determine whether or not somebody is mentally ill?
  • What level of mental illness constitutes "insanity"?
  • How do you prove somebody is (or was) insane?

The main reason that this concept is so confusing is that lawyer shows and press coverage of actual cases often don't clarify the distinction between insanity and mental illness. Mental illness and mental disorder are psychiatric concepts, while insanity is a cultural and legal concept. In a U.S. court of law (as well as courts in some other countries), insanity and mental illness are related conditions but they are by no means synonymous.

Mental illness at the time of the offense is a prerequisite for a not-guilty-by-reason-of-insanity ruling, but legal insanity is not simply a judgment of whether or not a person has a mental illness. The law varies from state to state, but in most courts that recognize the "insanity defense," someone is found to be legally insane if he or she meets one of three conditions:

  1. Because of a mental disorder, the defendant did not understand that what he or she was doing was illegal.
  2. Because of a mental disorder, the defendant did not know what he or she was doing.
  3. Because of a mental disorder, the defendant was compelled to commit the crime by an irresistible force.

So why is a person who meets these conditions not guilty of the crime? The reasoning is actually based on one of the most fundamental tenets of the justice system in the United States and many other countries: With most offenses, a person is only guilty of a crime if he or she actually intended to commit the crime. If you accidentally bump into somebody with your car, for example, you're not guilty of assault, but you would be guilty if you bumped into them intentionally, in exactly the same way. The action is the same, but you have not committed the crime because you didn't intend to (you may be guilty of a lesser crime, though -- e.g. reckless driving). Mental illness can alter a person's conception of reality so that he or she does not realize the criminal nature of his or her actions or has no choice but to commit the crime. When this is the case, certain courts believe the person lacks this element of intention necessary for criminal guilt.

To prove legal insanity, the defense must provide credible expert testimony that says how the defendant is (or was) mentally ill, and then explain why this sort of mental illness means that the defendant did not intend to commit a crime. The jury does not decide whether the defendant is mentally ill; it determines whether or not the defense's expert testimony has demonstrated this fact, and then decides whether or not this mental disorder meant that he or she did not intend to commit a crime. Mental illness alone is no defense -- A person who suffers from deep psychosis will still be considered guilty if he or she commits a crime intentionally.

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