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Are You (Legally) Insane?

Leah Tharakan

Edited by Ruth Yao and Vedanth Ramabhadran


Insanity defenses in criminal law have been a contentious and evolving concept in litigation, which serves as a crucial intersection between legal and mental health considerations– in many cases, drawing a fine line between the two. The insanity defense addresses cases where individuals, because of a severe mental disease or defect, were unable to appreciate the nature and quality or the wrongfulness of their acts which caused them to break the law; thus, such individuals may not bear the same level of culpability for their actions as individuals without mental defects [1]. While insanity defenses are generally accepted nationwide, except for Montana, Utah, and Idaho which only allow “guilty but insane” verdicts, they vary across state and federal jurisdictions [2]. Montana, Utah, Idaho, and Kansas are examples of states which have abolished the insanity defense, and their cases have raised questions about the Supreme Court and its willingness to allow states to punish defendants who lack moral responsibility. The differentiation in regulation demonstrates the intricate nature of insanity defenses and reflects the diverse cultural, legal, and societal perspectives on mental health and criminal responsibility [3]. 

Legal systems internationally grapple with the delicate balance between holding individuals accountable for their actions and recognizing the impact of mental health on their capacity to conform to the law.  The four prominent insanity standards– the M’Naghten Rule, the Irresistible Impulse (II) Test, the Durham Rule, and the Model Penal Code– vary from state to state depending on a state’s criminal laws and respective criminal justice system [3]. Each standard represents a distinct approach to assessing mental incapacity and its impact on criminal responsibility, as well as the ongoing challenge in creating an equilibrium upholding the principles of legal accountability and the nuances of mental health to ensure a just and equitable adjudication of cases involving individuals with profound psychiatric conditions.

The M’Naghten Rule

The M’Naghten Rule, also known as the “right-wrong” rule, roots from the 1843 case of Daniel M’Naghten, who shot and killed the Prime Minister’s secretary, Edward Drummond, after mistaking him for his intended target, Sir Robert Peel [4]. M’Naghten believed that he was possessed by Tories, a conservative political party, and further evidence demonstrated that he was distraught about the matter for a considerable period prior to the murder. It was understood from his demeanor after being coaxed, and finally tricked, into a “not guilty plea”. The judge acquitted the trial on grounds of insanity after hearing seven medical witnesses attest to M’Naghten’s delirium. M’Naghten was later forcibly committed to a mental facility where his story is now used as a foundational standard for determining legal insanity. The M’Naghten rule was a standard to be applied to the jury, after hearing medical testimony from prosecution and defense experts, and created the presumption of sanity unless the defense proved otherwise [1].

Central to the M’Naghten Rule is the notion of cognitive incapacity, focusing on the defendant’s awareness contrary to their ability to control conduct [3]. The rule denotes the defendant’s inability to comprehend the nature and quality of their actions at the time of the offense. To execute the defense, two elements are required: first, the defendant must be suffering from a mental defect at the time they commit the criminal act, and second, the defense must find that because of the mental defect, the defendant did not know either the nature and quality of the criminal act or that the act was wrong. In the case of the first element, a mental defect can simply be explained as the defendant being cognitively impaired to the point of not knowing wrongdoing, with some common examples being paranoia, schizophrenia, and psychosis. However, it is rare for the defendant to be completely oblivious to what they’re doing, so if defendants are claiming insanity, they opt to assert that they didn’t know the act was wrong. Jurisdictions then must make the distinction between what it means to be legally or morally wrong, the difference being wrong by law and wrong by societal standards.

The historical journey of the M’Naghten Rule underscores its legal development and the broader social evolution. Society’s growing sensitivity to mental health intricacies is mirrored in the rule’s emphasis on cognitive elements, demonstrating an ongoing commitment to the symbiotic relationship between legal standards and societal attitudes. Expanding the scope of insanity defenses to encompass a more comprehensive understanding of mental health can contribute to reducing stigma and promoting empathy towards individuals struggling with mental illness. In this way of recognizing the multifaceted nature of mental health conditions, legal standards can foster a more compassionate and inclusive society, where individuals are not solely judged based on their cognitive abilities but are understood and supported within the context of their mental health challenges.

The Irresistible Impulse (II) Test

While the M’Naghten Rule focuses on cognition, the Irresistible Impulse test focuses on the volitional components of insanity. Within the II test, a jury may find a defendant not guilty by reason of insanity where the defendant was compelled to commit an offense because of a mental disease. The test is aimed to address cases of people suffering from mental illnesses where individuals are incapable of self-control, such as paraphilias or mania [1]. The II test was first adopted by the Alabama Supreme Court in the 1887 case of Parsons v. State where it was found that a person may face a compulsion driven by the “duress of such mental disease.” The jury found that the mental disease had overpowered the defendant’s free agency [6].

There are two key elements which prove irresistible impulse, the first being the same as the M’Naghten Rule: the existence of mental illness. The second element is a causal link between diagnosed mental illness and the defendant’s incapacity to control their actions. This element is essential in demonstrating that mental illness directly contributed to the inability to exercise control. Despite the general notion that the II test provides an avenue for a more nuanced consideration of mental health in criminal proceedings, it is not without its critics [7]. Some opponents of the test argue that the definition of insanity under the II test is too broad and subjective, like that of the M’Naghten Rule. While the inclusion of the irresistible impulse criterion allows for a potentially expansive interpretation to establish clear boundaries between normal and pathological behavior, the ambiguity can also create a situation where individuals suffering from various mental disorders are easily excused for criminal acts, undermining the criminal justice system’s core goals of punishment and the prevention of future criminal activity. 

The II test, therefore, encapsulates the ongoing tension in insanity standards, attempting to balance compassion for individuals with mental disorders and the need for a clear and just legal framework. Its adoption and subsequent criticisms underscore the intricate challenges inherent in crafting standards that both acknowledge the impact of mental health on volitional control and prevent the unintended consequences of potentially ambiguous interpretations.

The Durham Rule

The Durham rule, or the product test, is used exclusively in the state of New Hampshire, and has been the established insanity defense in New Hampshire since the late 1800s, as the state adopted a version of the rule prior to receiving its current moniker [3]. The rule states “that an accused is not criminally responsible if his unlawful act was the product of mental disease or mental defect” [1]. Unlike the M’Naghten test, the Durham test doesn’t consider whether the defendant knew that their actions were wrong [8]. The rule stems from the case of Monte Durham, a 23-year-old who had been in and out of mental institutions and prisons since he was 17. After being convicted of breaking into homes in 1953, the federal appellate judge chose to use Durham’s case to reform the existing M’Naghten Rule because psychiatric experts at the time stated that the M’Naghten rule was based on “an entirely obsolete and misleading conception of the nature of insanity” [1]. The Durham rule was established to address the perceived inadequacies of the M’Naghten test, emphasizing a more contemporary understanding of mental health and its role in criminal responsibility by placing more significance on the expertise of psychological professionals’ findings over legal formalisms.

In a broader context, the Durham rule is based on proximate causation and comprises two essential elements. The first element, as the M’Naghten and II tests, mandates that the defendant must demonstrate the presence of a mental disease or defect.  The second element revolves around causation. Here, the focus shifts to determining whether the criminal conduct can be attributed as a direct result or "caused" by the identified disease or defect. This causative link serves as the impetus in the application of the Durham rule, introducing an element of proximate causation that forges the connection between the diagnosed mental condition and the commission of the unlawful act. The excusal of conduct under the circumstances hinges on establishing a compelling connection between the mental state of the defendant and the criminal behavior in question [3]. This dual-element structure not only reflects a departure from traditional insanity standards, but also underscores the Durham rule’s commitment to a more expansive, scientifically informed understanding of the complex interplay between mental health and criminal responsibility.

While the Durham rule appeared to improve upon the M’Naghten rule, the rule’s reliance on expert opinions gave way to concerns about conclusiveness and the erosion of the jury’s decision-making role. The rule faced problems because professionals held too much power compared to members of the jury who weren’t as educated on the matter and had a greater say over public welfare because of their social positions. Furthermore, a lack of continuity amongst professionals, specifically with regards to methodological approaches, contributed to the concerns. Additionally, issues from the M’Naghten and II tests resurfaced when terms like “mental disease or defect” were not clearly defined, leading to increased ambiguity. This resulted in inconsistent decisions because of the broad nature of the test, often letting people off the hook even if they understood and were in control of their actions. As a result, the D.C. circuit rejected the test in 1972 during the Brawner case, leaving New Hampshire to be the only state using the Durham rule.

The Durham rule's challenges with conclusiveness, professional discretion, and over-inclusivity have significantly restricted its adoption on the global stage within insanity defense. These inherent issues, such as the discretionary power granted to professionals and the lack of clear definitions for crucial terms, have limited its applicability. As a result, the Durham rule stands as an exception rather than the norm in international practices related to insanity defenses. Its limitations underscore the complex nature of crafting legal standards that balance the complex intricacies of mental health considerations with the need for consistent and transparent adjudications that a jury of one’s peers could reach without an overreach by unchecked professionals. 

Model Penal Code

The Model Penal Code was developed by the American Law Institute in 1962 to modernize and standardize criminal statutes across jurisdictions [9]. The Model Penal Code introduces a multifaceted approach to the determination of legal insanity, departing from the more rigid standards such as the M'Naghten Rule and the II Test. Under the Model Penal Code, a defendant is considered not guilty by reason of insanity if diagnosed with a relevant mental defect and, at the time of the crime, was unable to appreciate the criminality of their conduct or conform their behavior to the requirements of the law. The differentiating factor between the Model Penal Code and other insanity defenses is that the Model Penal Code covers gaps in other tests, such as its broader scope, encompassing aspects beyond the rigid confines of the M’Naghten Rule. It incorporates the concept of volition, as addressed in the II test, and it is notably utilized by states that don’t adhere to the M’Naghten Rule. The Model Penal Code does not allow psychopaths and sociopaths from employing the insanity defense, further distinguishing its approach.

Despite the Model Penal Code's commendable effort to synthesize various aspects of traditional insanity tests, it is not immune to criticism. The provision's use of terms such as "appreciate" introduces an element of subjectivity that, while intending to encompass a range of mental disabilities, might lead to challenges in consistent application. Likewise, the flexibility given to legislatures in choosing between the terms "criminality" and "wrongfulness" introduces potential ambiguity, leaving room for interpretation that might vary across jurisdictions. Additionally, the Model Penal Code's exclusion of psychopaths and sociopaths from the insanity defense sparks debates about fairness and the potential impact on individuals with severe personality disorders who may also experience mental health challenges.

The Model Penal Code represents a significant leap forward in the evolution of insanity standards, addressing both cognitive and volitional aspects of criminal responsibility. It’s an attempt to reconcile traditional tests and provide flexibility to legislatures to adapt legal frameworks to contemporary understandings of mental health. However, the inherent subjectivity in certain language choices and the exclusion of psychopaths and sociopaths present challenges and invite ongoing discourse. While the Model Penal Code marks a progressive step in the refinement of insanity standards by incorporating cognitive and volitional elements, its implementation necessitates careful consideration of subjective language nuances and the ethical implications of excluding certain individuals from the insanity defense.


While insanity defenses have long been a loophole that defendants could exploit to evade punishments for their actions, it is evident that the use of the defenses are the first step in establishing mental health reinforcements to the criminal justice system. Critics allege that some criminal defendants may strategically exaggerate symptoms of mental disorders to rationalize criminal behavior, which has cast significant doubt on the scientific credibility of this defense. Comparing the four insanity standards– the M’Naghten rule, the Irresistible Impulse test, the Durham rule, and the Model Penal Code– reveals diverse approaches to assessing mental incapacity and criminal responsibility. The M’Naghten Rule, with its focus on cognitive elements, has provided a foundation for many jurisdictions, emphasizing an individual's understanding of right and wrong at the time of the crime. The Irresistible Impulse Test introduces a volitional component, allowing for the consideration of cases where individuals, despite recognizing the wrongfulness of their actions, are compelled to commit offenses due to mental illness. In contrast, the Durham Rule, while seen as an innovative approach at the time, faced challenges such as conclusiveness, professional discretion, and over-inclusivity, and ultimately falling out of favor. The Model Penal Code, as a comprehensive and modernized standard, incorporates both cognitive and volitional elements, aiming to reconcile traditional tests and adapt to contemporary understandings of mental health.

While these insanity defenses serve as crucial components in the legal system, the persistent debates and critiques underscore the balance required in instituting standards that genuinely reflect the intersection of mental health and criminal responsibility. As the legal landscape continues to evolve, the ongoing conversation surrounding mental health remains essential for refining legal frameworks, such as insanity defenses, with scientific advancements while mitigating the potential for exploitation or manipulation by defendants.


[1] Insanity Defense, LII / Legal Information Institute, (last visited Mar 6, 2024).

[2] Insanity Defense, Findlaw, (last visited Mar 6, 2024).

[3] The Insanity Defense, University of Minnesota (2015), (last visited Mar 6, 2024).

[4] The Insanity Defense in Criminal Law Cases, Justia (2018), (last visited Mar 6, 2024).

[5] Suresh Bada Math, Channaveerachari Naveen Kumar & Sydney Moirangthem, Insanity Defense: Past, Present, and Future, 37 Indian Journal of Psychological Medicine 381 (2015). 

[7] On Behalf of Mayer Law Office & LLC, The Irresistible Impulse Test: What You Should Know | Mayer Law Office, LLC, (2021), (last visited Mar 7, 2024).

[8] Durham test, LII / Legal Information Institute, (last visited Mar 7, 2024).

[9] The “Model Penal Code” Test for Legal Insanity, Findlaw, (last visited Mar 7, 2024).

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