insanity defense

Primary tabs


The insanity defense refers to a defense that a defendant can plead in a criminal trial. In an insanity defense, the defendant admits the action but asserts a lack of culpability based on mental illness. 

The insanity defense is classified as an affirmative defense, rather than a partial defense.

Insanity & Diminished Capacity 

Although the defense known as "diminished capacity" bears some resemblance to the "reason of insanity" defense (in that both examine the mental competence of the defendant), there are significant differences between them. While "reason of insanity" is a full defense to a crime -- that is, pleading "reason of insanity" is the equivalent of pleading "not guilty" -- "diminished capacity" is merely pleading to a lesser crime. A diminished capacity defense can be used to negate the element of intent to commit a crime.

One of the most famous uses of the insanity defense in the U.S. came in United States v. Hinckley, concerning the assassination attempt against then-President Ronald Reagan.


An important procedural corollary to the insanity defense involves the establishment of legal competency, otherwise known as competence, to stand trial. In accordance with due process requirements, a criminal defendant cannot stand trial if they are deemed legally incompetent. As articulated by the Supreme Court in Dusky, a defendant is incompetent if they are incapable of rationally communicating with their attorney or rationally comprehending the nature of the proceedings against them. A defendant may move at any time for a hearing to determine competency, which involves the submission of supporting evidence and some form of a psychological evaluation. The threshold for establishing competency is often identified as notoriously low. So long as a defendant is deemed incompetent, the insanity defense becomes moot as the defendant cannot stand trial. The case of People v. William Freeman (1847) was an important point in the formulation of the insanity defense as the Court held in that case that, even if a defendant is held to be competent enough to stand trial, they can still present evidence during the trial to support their defense of insanity.

The ''M'Naghten Rule''

The first famous legal test for insanity came in 1843, in the M'Naghten case. Englishman Daniel M'Naghten shot and killed the secretary of the British Prime Minister, believing that the Prime Minister was conspiring against him. The court acquitted M'Naghten "by reason of insanity," and he was placed in a mental institution for the rest of his life. However, the case caused a public uproar, and Queen Victoria ordered the court to develop a stricter test for insanity.

The "M'Naghten rule" was a standard to be applied by the jury, after hearing medical testimony from prosecution and defense experts. The rule created a presumption of sanity unless the defense proved "at the time of committing the act, the accused was laboring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing or, if he did know it, that he did not know what he was doing was wrong."

This analysis focuses on an actor's cognition. The test is bifurcated into two components, each of which is individually sufficient to substantiate an insanity defense. First, a defendant is deemed insane if they were incapable of knowing what they were doing at the time committing the object offense. This conclusion comports with criminal law's fundamental conception of culpability. A defendant is not culpable for an act that, because of a psychological infirmity, they did not know they were committing.

The second component of the test looks to determine if the defendant knew that their actions were wrong. Here, even if the defendant knew what they were doing, they are deemed insane where they were incapable of recognizing the wrongfulness of the action committed. A paradigmatic example of this analysis involves deific decrees. In these cases, the defendant is often found insane on the grounds that, because "God" commanded the defendant to act,they were unable to recognize the wrongfulness of the act that was carried out. 

Various legal commentaries have identified theoretical issues within the M'Naghten framework. For example, a scholarly debate exists addressing whether the "wrongfulness" central to the M'Naghten analysis comprises tenets of legality or morality. Another prominent criticism takes objection to the categorical approach the M'Naghten test employs. By focusing exclusively on cognitive incapacity, the M'Naghten test is not well suited for treating more nuanced forms of psychological disorders, particularly those involving volitional impairment. Traditionally, the M'Naghten test has been associated with schizophrenia and psychotic disorders.

The M'Naghten rule became the standard for insanity in the United States and the United Kingdom and is still the standard for insanity in almost half of the states.

The "Irresistible Impulse" Test

In contrast to the emphasis on cognition central to the M'Naghten test, the "Irresistible Impulse" test focuses on the volitional components of insanity. Various courts have struggled to address criminal defendants who, while comprehending the wrongfulness of their actions, are incapable of self-control because of a mental disease or defect. To levy punishment against a defendant unable to control their actions appears at odds with the preeminent tenets of criminal justice. The move towards volition alleviates this tension. Under the "Irresistible Impulse" test a jury may find a defendant not guilty by reason of insanity where the defendant was laboring under a mental disease or defect that compelled them to commit the object offense. This test is well-suited for people suffering from mania and/or paraphilias.

While treating a genuine issue within the M'Naghten framework, the "Irresistible Impulse" test creates several practical concerns. First, unlike the cognitive prong of the insanity defense, the volitional component of insanity is substantiated by a less robust scientific literature. Consequently, evaluating the veracity of a defendant's claim becomes more difficult in the absence of unequivocal scientific findings. Moreover, the "Irresistible Impulse" test may be over-inclusive. Defendants laboring under psychological conditions, which, while genuine, do not completely inhibit self-control, may be exonerated of criminal liability

The Durham Rule (The "Product" Test)

Monte Durham was a 23-year-old who had been in and out of prison and mental institutions since he was 17. He was convicted of housebreaking in 1953, and his attorney appealed. Although the district court judge had ruled that Durham's attorneys had failed to prove he didn't know the difference between right and wrong, the federal appellate judge chose to use the case to reform the M'Naghten rule. Citing leading psychiatrists and jurists of the day, the appellate judge stated that the M'Naghten rule was based on "an entirely obsolete and misleading conception of the nature of insanity." The court of appeals overturned Durham's conviction and established a new rule. The Durham rule states "that an accused is not criminally responsible if his unlawful act was the product of mental disease or mental defect." 

The implementation of this test was initially seen as a progressive development. Specifically, the Durham rule moved away from legal formalisms and emphasized scientific psychological evaluations and evidence. This approach emphasized expert testimony and largely left the jury to follow the professional opinions provided. So long as a professional concluded that the defendant was subject to a mental disease, a finding of insanity likely followed.  

Problems quickly emerged, however, and the Durham test fell out of favor. First, the test revealed itself to be frequently conclusory and deprived the jury of their decision-making role. A finding of insanity was left to the discretionary findings of trained professionals who were largely unrestricted in their methodological approach. The lack of any clear definition for essential terms like "mental disease or defect" exacerbated this issue and led to inconsistency as different professionals came to disparate conclusions. Moreover, the test proved over-inclusive. Under the "product" approach, defendants could be found not guilty by reason of insanity even where they understood and had control over their actions at the time of the offense. For these individuals, punishment may be more appropriate as its deterrent effect remains intact. Consequently, the same D.C. circuit that adopted the test in 1954, rejected the test in 1972 when deciding the Brawner case. New Hampshire is now the only jurisdiction that employs a test similar to the Durham rule.

The Model Penal Code

In 1972, in an attempt to modernize the legal standard for insanity, the American Law Institute, a panel of legal experts, developed a new rule for insanity as part of the Model Penal Code. This rule, found in §4.01 of the Code, says that a defendant is not responsible for criminal conduct where (s)he, as a result of mental disease or defect, did not possess a "substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of the law."

Section 4.01 of the Code represents a concerted effort to reconcile the various details and emphases present in the traditional tests for insanity. First, the language, "appreciate," addresses the cognitive component central to the M'Naghten analysis. Unlike the oft rigid M'Naghten test, however, the "appreciate" language of §4.01 is broad and intended to realistically address the graded nuances of mental disabilities. Moreover, the rule allows the legislature to choose between the language of "criminality" or "wrongfulness." This presentation allows legislatures to choose between a more legalistic conception of wrong, in the form of "criminality," or a broader, morally infused understanding, in the form of "wrongfulness." The evaluation of "wrongfulness" or "criminality" is complex and intricate. Complications arise, for example, when considering persons who, although knowing society would condemn their acts as wrong or criminal, believe that this would not be the case if society knew what they were "aware" of.

The second component of §4.01 comprises a volition-based analysis. This aspect of the Model Penal Code's insanity standard reflects the theoretical foundation supporting the "Irresistible Impulse" test. Here, the Code dictates that criminal liability is unjustified where a defendant could not  "conform his conduct to the requirements of the law." This emphasis on conformity looks to provide for those persons cognizant of their wrongful act, but unable, because of some mental disease or defect, to control themselves. The inclusion of this volitional analysis, alongside a cognitive analysis, represents the progressive nature of the insanity standard developed in the Model Penal Code.

Finally, of particular importance is §4.01(2) of the Code. There, the use of the insanity defense for psychopaths and sociopaths is expressly proscribed.

Comprehensive Crime Control Act

In 1984, Congress passed, and President Ronald Reagan signed, the Comprehensive Crime Control Act. The federal insanity defense now requires the defendant to prove, by "clear and convincing evidence," that "at the time of the commission of the acts constituting the offense, the defendant, as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of his acts" (18 U.S.C. § 17). This is generally viewed as a return to the "knowing right from wrong" standard. The Act also contained the Insanity Defense Reform Act of 1984, 18 U.S.C. § 4241, which sets out sentencing and other provisions for dealing with offenders who are or have been suffering from a mental disease or defect.

Other Approaches

In addition to these seminal understandings of the insanity defense, some legal theorists have endorsed alternative conceptions of the insanity defense in an attempt to address various weaknesses often identified. The integrationist approach, for example, does away with the insanity defense as a unique defense to criminality and evaluates individual defendants under traditional exculpatory defenses, like duress or necessity. Yet another modification comes from the abolitionist perspective. Under this model, some scholars, contending that social benefit can be derived from the punishment of persons often exculpated by the insanity defense, have suggested the eradication of the insanity defense in its entirety.

[Last updated in June of 2023 by the Wex Definitions Team]