What is the Insanity Defense


The insanity defense is often featured in crime dramas, with a defendant claiming mental illness to try to avoid being convicted of a crime. However, the insanity defense is difficult for defendants to prove, and even raising a defense of insanity or diminished capacity can have serious implications for a defendant if the defense is rejected by a jury.

What is the Insanity Defense

The concept of insanity is a legal term, not a clinical term, and thus there is no clinical means of having a defendant in a criminal case declared sane or insane by mental health professionals. The concept of insanity is now typically referenced as criminal responsibility, whether or not the defendant is responsible for criminal conduct that occurred while the defendant suffered from a mental illness or impairment.

Most states recognize a form of insanity or diminished capacity defense. The defense is not recognized in the states of Idaho, Kansas, Montana or Utah; but under narrow circumstances evidence of mental illness may nonetheless be admissible to show that a defendant was incapable of forming intent to commit the crime for which he is being prosecuted.

Possible variations of the insanity defense include:

  • Lack of Criminal Responsibility - The defendant argues that due to a mental condition that existed at the time of the alleged offense, he was incapable of knowing that his actions were wrongful and should thus be excused from criminal responsibility for those acts. Depending upon the state, this defense usually takes one of the following two forms:

    1. A defendant must prove that at the time of committing the criminal act, as a result of mental disease or defect he was laboring under such a defect of reason that he did not know the nature and quality of the act he was committing or, if he did know it, that he was not capable of understanding that his act was wrong; or

    2. A defendant must prove that at the time of committing the criminal act, as a result of mental disease or defect he lacked substantial capacity either to appreciate the criminal nature of his conduct, or to conform his conduct to the requirements of the law.

    Although the two standards are similar, the second version is slightly broader in application.

  • Temporary Insanity - Although not technically different than a broader claim that the defendant was not criminally responsible, under this variant the it is asserted that the defendant is not criminally responsible due to a short-term mental crisis or disorder that has since resolved.

  • Automatism or Irresistible Impulse - A claim that the defendant was subject to a condition that did not prevent him from distinguishing right from wrong, but that prevented him from exercising control over his actions such that he was unable to prevent himself from committing a crime.

What Happens if You Plead Not Guilty by Reasons of Insanity

Under typical state law, a defendant who pleads not guilty by reason of insanity may be ordered to undergo evaluation by state experts, to assess whether the defendant suffers any ailment or condition at the time of a criminal offense that might affect his responsibility for committing that offense. The defendant gives up his right to dispute the facts of the charge, and is limited in his defense to attempting to prove that he was not criminally responsible. As a consequence, if the jury rejects the defendant's defense, the defendant will be convicted of the crime and sentenced in the same manner as any other defendant.

A small number of states permit a defendant to raise an insanity defense after an initial trial that determines his guilt, but most states now require a guilty plea. A defendant who raises this defense will present expert testimony from mental health professionals, and must participate in examinations by prosecution experts. The competing opinions of the experts are presented in court, and the judge or jury must decide whether the defendant was or was not criminally responsible based upon that testimony and any supporting evidence.

The insanity defense is rarely successful. When a defendant is not convicted as a result of raising the defense it's usually because:

  • The prosecutor agrees to accept a plea of not guilty by reason of insanity as part of a plea bargain; or

  • The defendant spends so much time in pretrial detention, in an effort to treat his mental illness to the degree that he's competent to stand trial, that the charge is dismissed based upon his having served most or all of the maximum sentence for the alleged offense.

If a defendant is found not guilty by reason of insanity, the defendant may be sentenced to a mental health facility for treatment. The quality of mental health treatment for people within the criminal justice system is often questionable. A large number of defendants who are committed for mental health treatment instead of being sentenced to prison end up spending considerably more time in detention, as compared to the amount of time they would have spent in prison had they simply entered a guilty plea.

What is a Plea of "Not Guilty but Mentally Ill"

Some states allow a defendant to enter a guilty plea to a crime, without attempting to prove that they are not criminally responsible, asking that their mental illness be taken into consideration during sentencing. As a result of this form of plea, the defendant may be able to introduce more mitigating evidence, based upon mental health issues, than would be allowed in a typical criminal sentencing.

In states that allow this form of plea, if the court finds evidence of mental illness to be credible, the court may sentence the defendant to mental health treatment. Once the treatment is complete, the defendant must normally serve the remainder of his sentence in a standard correctional facility or, if eligible, be released on standard probation or parole.

Copyright © 2016 Aaron Larson, All rights reserved. No portion of this article may be reproduced without the express written permission of the copyright holder. If you use a quotation, excerpt or paraphrase of this article, except as otherwise authorized in writing by the author of the article you must cite this article as a source for your work and include a link back to the original article from any online materials that incorporate or are derived from the content of this article.

This article was last reviewed or amended on Oct 19, 2017.