When can a defendant win an acquittal on grounds of insanity?

The insanity defense is based on the principle that punishment is justified only if the defendant is capable of controlling his or her behavior and understanding that what he or she has done is wrong. Because some people suffering from a mental disorder are not capable of knowing or choosing right from wrong, the insanity defense prevents them from being criminally punished.

Despite its ancient origins (England, 1505), the insanity defense remains controversial. Victim-oriented critics point out that a person killed by an insane person is just as dead as a person killed by someone who is sane, and argue that people should be punished for the harm they cause regardless of their mental state. Critics also question the ability of psychiatrists, judges, and jurors to determine whether a person suffers from a mental disorder, and to link mental disorders to the commission of crimes.

The insanity defense is an extremely complex topic; many scholarly works are devoted entirely to explaining its nuances. Here are some major points of interest:

Despite popular perceptions to the contrary, defendants rarely enter pleas of not guilty by reason of insanity. On the few occasions that the defendant does raise it, judges and jurors rarely support it.

Because neither the legal system nor psychiatrists can agree on a single meaning of insanity in the criminal law context, various definitions are employed. The most popular definition is the McNaghten rule, which defines insanity as the inability to distinguish right from wrong. Another common test is known as an irresistible impulse: A person who acts out of an irresistible impulse knows that an act is wrong, but because of mental illness, cannot control his actions.

Defendants found not guilty by reason of insanity are not automatically set free. They are usually confined to a mental institution, and not released until their sanity is established. These defendants can spend more time in a mental institution than they would have spent in prison had they been convicted.

An insanity defense normally rests on the testimony of a psychiatrist, who testifies for the defendant after examining him and his past history, and the facts of the case. Courts appoint psychiatrists at government expense to assist poor defendants who cannot afford to hire their own psychiatrists.


What happens if a defendant is judged incompetent to stand trial?

Aside from insanity as a defense to criminal charges, the question may arise as to whether a defendant is mentally capable of facing a trial. Defendants cannot be prosecuted if they suffer from a mental disorder that prevents them from understanding the proceedings and assisting in the preparation of their defense. Based on a defendant’s unusual behavior, a judge, prosecutor, or defense attorney may ask that trial be delayed until the defendant has been examined and her ability to understand the proceedings has been determined in a court hearing. If a judge finds that a defendant doesn’t understand what’s going on, the defendant will probably be placed in a mental institution until her competence is re-established. At that time, the trial will be held.

Can a defendant go free because he was drunk or high on drugs when he committed a crime?

Defendants who commit crimes under the influence of drugs or alcohol sometimes argue that their mental functioning was so impaired that they cannot be held accountable for their actions. Generally, however, voluntary intoxication does not excuse criminal conduct. People know (or should know) that alcohol and drugs affect mental functioning, and thus they should be held legally responsible if they commit crimes as a result of their voluntary use.

Some states allow an exception to this general rule. If the defendant is accused of committing a crime that requires what’s known as specific intent (intending the precise consequences, as well as intending to do the physical act that leads up to the consequences), the defendant can argue that he was too drunk or high to have formed that intent. This is only a partial defense, however, because it doesn’t entirely excuse the defendant’s actions. In this situation, the defendant will usually be convicted of another crime that doesn’t require proof of a specific intent-for example, assault with a deadly weapon instead of assault with the intent to commit murder.