Insanity Defense

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The defense of insanity is an affirmative defense that exculpates the defendant from any criminal liability because, at the time of the crime, the person did not appreciate the nature, quality, or wrongfulness of his actions. A defendant who claims defense by reason of insanity normally must undergo a psychological examination before putting forth the defense. Use of the insanity defense tends to be rare and sustaining the defense at trial is very difficult. Putting forth an insanity defense might be said to be equivalent to pleading ‘not guilty by reason of insanity’ which, if successful, will result in the defendant being committed to a psychiatric institution instead of prison for an indeterminate period. Many jurisdictions allow for a defense by diminished capacity, which serves as a mitigating factor to reduce charges or sentences.

Allowing an insanity defense is based on the theory that conviction and punishment are justified only if the defendant deserves them, and it would be unfair to punish someone who is so mentally disturbed that he lacks responsibility as a moral agent. The legal definition of insanity is very different from the psychiatric definitions of mental illness and should not be used interchangeably.

The insanity defense has historical roots in common law dating back to 13th Century England when “complete madness” was established as a defense to criminal charges. The test evolved over time and by the 18th Century had evolved into the “wild beast” test, wherein a person deprived of his understanding and memory so as not to be any more aware of his actions than a wild beast could be excused from criminal liability for his conduct.[1]

International Law

The Rome Statute of the International Criminal Court provides for an insanity defense. Article 31(1)(a) provides that a person shall not be criminally responsible if, at the time of that person's conduct:

"The person suffers from a mental disease or defect that destroys that person's capacity to appreciate the unlawfulness or nature of his or her conduct, or capacity to control his or her conduct to conform to the requirements of law;"[2]

The Trial Chamber of the International Criminal Tribunal for Yugoslavia has held that the mental disease defense is only admissible in two circumstances: (1) in case of impairment of the accused's capacity to appreciate the unlawfulness of or the nature of his conduct; or (2) in case of an impairment to control his conduct in order to conform to the requirements of the law. The statute however requires a destruction of mental capacity for the Article 31(1)(a) insanity defense to apply.[3]

Specific Country Applications

Sri Lanka

Sri Lanka Penal Code § 77 provides that ”[n]othing is an offence which is done by a person who, at the time of doing it, by reason of unsoundness of mind, is incapable of knowing the nature of the act, or that he is doing what is either wrong or contrary to law.”[4] The Penal Code allowance for an insanity defence takes the approach that a mental incapacity in the defendant negates the requisite mens rea to commit a criminal act and can thus exculpate the defendant from criminal liability.

When the “unsound mind” defense is raised, the court conducts a medical examination of the defendant and further engages in a preliminary inquiry to determine the defendant’s mental status. If convicted, the verdict must list whether or not the person of unsound mind actually committed the act in question and will be examined at least once every sixth months while being held in custody of a prison or mental institution to ascertain his mental state. A person of unsound mind can also be remanded to the custody of a caretaker home instead of the prison if approved by the prison commissioner.[5]

Additionally, the court of appeals may quash a guilty verdict against a person it feels to be of unsound mind per the Penal Code § 77 definition and order that person into safe custody instead of prison.[6]

United States

Currently, the federal government and 48 states have some form of insanity defense available to criminal defendants. The rules of application for the insanity defense, however, will vary by jurisdiction. Similarly, the consequences of mounting a successful insanity defense vary by jurisdiction, but on average a defendant found guilty by reason of insanity is committed to a mental institution for twice as long as a defendant who is found guilty and sent to prison. Very few acquitted insanity defendants are given supervised release, and the defense by no means function as an escape clause from imprisonment.[7]

On average less than 1 defendant in 100 actually raises the insanity defense, and success rates of all insanity defenses is an extremely low 0.26%. Some studies show that as many as 70% of defendants withdrew their insanity defense after a state-appointed expert found them to be legally sane. In general, mock juries tend to form personal opinions about the person claiming the insanity defense and do not always understand the varying legal definitions at play. Recognizing the rarity of the insanity defense, some courts allow for a verdict of “guilty but mentally ill,” which recognizes that a defendant’s mental illness played a role in his crime without entirely causing it. A verdict of “guilty but mentally ill” results in the defendant’s incarceration but also the provision of treatment for his mental illness.[8]

While most states will not allow a defendant to withdraw or change his insanity plea once entered, others have allowed a change of defense on habeas corpus.[9] One court went so far as to hold that an insanity defense could not be imposed upon an unwilling defendant if an intelligent defendant voluntarily wishes to forgo the defense.[10]

The insanity defense is a controversial topic and has gone through several formulations throughout its history in U.S. courts, including the following:

  • The M’Naghten Rule

Drawn from British common law, the M’Naghten rule has been extremely influential on U.S. insanity defense standards for the past two centuries. To this day, many states still employ some version or elements of the M’Naghten rule in determining a defendant’s sanity.

In 1843, Daniel M’Naghten murdered the secretary of the Prime Minister of England in a failed attempt to assassinate the prime minister himself, believing that the prime minister was to blame for his life’s troubles. The trial court acquitted M’Naghten by reason of insanity, and the House of Lords developed from there the M’Naghten Rule, (1843) 10 C & F 200. The M’Naghten rule state that a defendant will not be held responsible for his actions if, due to his mental disease or defect, he did not know that his act would be wrong or did not understand the nature and quality of his actions.[11]

  • Irresistible Impulse Test

Many jurisdictions later added a volitional element to the M’Naghten cognitive test for insanity, creating the “irresistible impulse test.” Under the irresistible impulse test, a defendant is not guilty by reason of insanity if the defense establishes that, because of a mental illness, the defendant was unable to control his actions or conform his conduct according to the law. The irresistible impulse test allows for a defendant to recognize that his conduct is wrong but excuses criminal liability where he is unable to control his actions due to some mental disease or defect.

  • The Durham Rule

The M’Naghten rule was used in the U.S. until the case of Durham v. United States. Lauded by the mental health community as progressive because it allowed the involvement of psychologists in the insanity determination, the Durham rule excused a defendant “if his unlawful act was the product of mental disease or mental defect.”[12] Generally seen as an overly lenient standard of legal insanity, the rule has since been explicitly rejected in 22 states and is only used in New Hampshire.[13]

  • Model Penal Code

After a reassessment of the insanity defense, the Model Penal Code formulated a compromise between the narrow M’Naghten test and the more generous Durham rule.[14] Model Penal Code § 4.01 sets forth the “substantial capacity” standard, which excuses a person from criminal liability if at the time of his criminal action he lacked the substantial capacity either to appreciate ”the criminality [wrongfulness] of his conduct or to conform his conduct to the requirements of the law” as a result of some mental disease or defect.[15] The Model Penal Code “substantial capacity” test takes into account both the cognitive and volitional capacity of insanity, and was adopted by more than half of the states and all but one Federal Circuit.

  • Insanity Defense Reform Act

In 1981, John W. Hinckley, Jr. attempted to assassinate President Ronald Regan. Hinckley was acquitted of all charges by reason of insanity under the Model Penal Code’s “substantial capacity” test, leading to a public outcry that the U.S. was too lenient on defendants attempting to assert the insanity defense. The result of a compromise between hard-line legislators looking to abolish the insanity defense and psychiatric and legal professionals looking to retain it was the Insanity Defense Reform Act, which shifted the burden of proof onto the defense to show the defendant’s insanity.[16] The law additionally added a number of procedural barriers to claiming an insanity defense, and tightened the standard of proof to a showing that the mental disease or defect was “severe.” Congress eliminated the volitional prong of the test that had allowed a defendant to merely show he could not control his behavior.

While some states followed suit and changed their insanity defense laws to put the burden of proof on defendants as well, others created stricter standards of proof and a handful eliminated the insanity defense altogether.

See Defenses


  3. Geert-Jan G. J. Knoops, Defenses in Contemporary International Criminal Law, p. 140-142, available at, citing Prosecutor v. Delalic, Case No. IT-96-21, ICTY Trial Chamber Judgment, para 283 (Nov. 16, 1998)
  4. Penal Code § 77 (No. 2 of 1883), available at
  5. Code of Criminal Pocedure § 374 – 386 (No. 2 of 1979), available at
  6. Code of Criminal Procedure § 338
  9. Jerome Nwokike, Federal Insanity Acquittees: Person Found Not Guilty by Reason of Inasnity May Not Attack His Successful Insanity Defense in Habeas Petition, J. AM. ACAD. PSYCHIATRY LAW, 33:1:126-128 (2005), available at
  10. For more information, see
  12. Durham v. United States, 214 F.2d 862 (D.C. Cir. 1954), available at
  15. Model Penal Code § 4.01, available at
  16. Insanity Defense Reform Act (Insanity Act), 18 U.S.C. § 17 (1988), available at