The Insanity Defense: From Fundamental Legal Principle to Legal Fiction

In Kahler, the Court did not merely uphold Kansas' interpretation of a vital protection; it allowed Kansas to, in effect, eviscerate the protection entirely.

Photo: The Federalist Society, Courtesy of Google Images

The U.S. Supreme Court recently shook the foundation of our criminal justice system in their 6-3 decision of Kahler v. Kansas by holding that there is no Constitutional requirement for States to provide an insanity defense for a defendant.

The Supreme Court’s holding is so jaw-dropping because the insanity defense is a fundamental principle of justice developed even before the formation of the United States of America.  The holding is incredibly disappointing, given the tremendous public support the United States has exponentially shown for mental illness.

Relevant Cases

A review of the relevant case law begins with the famous ruling in M’Naghten’s Case (1843).  The defendant was charged with murder after he mistakenly shot the secretary to the Prime Minister, missing his primary target, the Prime Minister, Sir Robert Peel.  At trial, the defendant introduced extensive expert and lay evidence to show that he was obsessed with delusions and suffered from acute insanity.  The Court declared that a defendant would be found not guilty by reason of insanity if, as a result of mental disease, he did not know or appreciate the nature or quality of the criminal act he committed or did not know that what he was doing was wrong.

More recently, in the 2006 U.S. Supreme Court decision of Clark v. Arizona, the Court debated the insanity defense.  In this case, the defendant was charged with murder after shooting and killing a police officer who had pulled the defendant over.  At the defendant’s bench trial, the defendant pleaded not guilty by reason of insanity.  He sought to introduce evidence by lay and expert witnesses of his undisputed mental illness, paranoid schizophrenia, to show that he did not intend to shoot the officer and did not know what he was doing.   The Supreme Court held that a State’s determination that expert testimony about a defendant’s mental incapacity due to a mental disease or defect is admissible only for its bearing on an insanity defense is barred on the element of mens rea, and does not violate Due Process.

Even more recently, in Kahler v. Kansas, the defendant was charged with capital murder after shooting and killing four family members.  During the 2020 Kansas Supreme Court bench trial, the defendant argued that Kansas’ insanity defense violates Due Process because it permits the State to convict a defendant whose mental illness prevented him from distinguishing right from wrong.  The Kansas Supreme Court held that Due Process does not require States to adopt an insanity test that turns on a defendant’s ability to recognize that their crime was morally wrong.

The Undisputed History of the Insanity Defense

The principles surrounding the insanity defense have been echoed since the time of Aristotle.  In fact, the insanity defense was used as an excuse in the third century by the Roman lawyer, Modestinus.  He wrote, “if a madman commit homicide he is not covered by the Cornelian Law [which laid down the legal consequences] because he is excused by the misfortune of his fate.”  This early concept is known as “divine punishment” and focuses on the idea that a “madman,” by his disease, has been punished enough.

As the insanity defense developed through the Medieval and Renaissance periods, there was a shift to an idea that a defendant must understand the difference between good and evil.  According to Bernard Diamond in 1233 the “good and evil” test was explained as, man’s freedom “is restrained in children, in fools, and in the witless who do not have reason whereby they can choose the good from the evil.”  Later in the 13th century, lawyer, Henry de Bracton believed that “a crime is not committed unless the will to harm [voluntas nocendi] be present.”

The “wild beast” test and the “right and wrong” test followed the “good and evil” test moving up into the 19th century.  The “wild beast” test was short lived due to the concern that the term wild beast was misconstrued.  The test, developed in Rex v. Arnold, was that “a mad man . . . must be a man totally deprived of his understanding and memory, and doth not know what he is doing, no more than a brute, or a wild beast. Such a one is never the object of punishment.”   The term wild beast was often interpreted as a violently wild, ravenous beast.  Lawyers tended to not focus on the lack of intellectual ability of woodland animals, which is what the term wild beast was typically used for at that time.

The “right and wrong” test was stemmed from the “good and evil” test and was created in Bellingham’s Case in 1812.  The jury was instructed to decide whether the defendant “had sufficient understanding to distinguish right from wrong.”  The test remained very similar to the “good and evil” test until it was altered by Regina v. Oxford in 1840.   The modified “right and wrong” test presented the question of whether the defendant, “from the effect of a diseased mind” knew that the act was wrong.  Further, it must be asked whether “he was quite unaware of the nature, character, and consequences of the act he was committing.”  This test has gone on to be known as the precursor of the M’Naghten test.

The landmark English ruling of M’Naghten’s Case in 1843 held that:

to establish a defense on the ground of insanity it must be clearly proved that at the time of the committing of the act the party accused was laboring under such a defect of reason, from disease of mind, and 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 test for insanity gained popularity in American decisions and statutes with a two-pronged analysis focusing on cognitive and moral capacity.  Cognitive capacity describes whether the mental illness has left a defendant unable to understand what he was doing when he committed a crime.  In contrast, moral capacity refers to whether the mental illness has left a defendant unable to understand that his action was wrong.

Under Kansas law, a defendant may only raise the defense of mental illness to show that he or she “lacked the culpable mental state required as an element of the offense charged.” 

While showing a lack of mens rea is the only way to produce an acquittal, Kansas law does allow a defendant to use evidence of mental illness to argue for a lessened punishment at sentencing.  Ultimately, Kansas allows evidence of mental illness showing cognitive incapacity, but not moral incapacity.

To better understand this, consider a hypothetical, including the prosecution of two defendants for murder.  In the first prosecution, the defendant shot and killed another person, and at trial, evidence shows that, as a result of severe mental illness, he thought the victim was a zombie.  The second prosecution is the same except that the evidence at trial proves that, as a result of severe mental illness, the defendant believed that a zombie ordered him to kill the victim.

Under Kansas’ rule, the first defendant would be free from prosecution because the evidence of his mental illness would show that he lacked the mens rea to kill another person.  The second defendant would be subject to prosecution by Kansas’ rule because he intended to kill another person.  Kansas’ rule completely ignores the fact that the second defendant believes he is not only communicating with a zombie but also following its decrees.

The Supreme Court’s Decision

Writing for the majority, Justice Kagan states,

a state rule about criminal liability—laying out either the elements of or the defenses to a crime—violates due process only if it ‘offends some principle of justice so rooted in the traditions and conscience of our people as to be ranked as fundamental.’

Justice Kagan also recognizes that the Court’s “primary guide in applying th[is] standard is ‘historical practice.’”  Contrary to the Court claiming to use historical practice as its guide, the majority carelessly dismisses the fact that historically forty-five states, the federal government, and the District of Columbia have adopted an insanity defense that allows some form of inquiry into the blameworthiness of the accused.

Further, the Majority in Kahler argues that Kansas’ rule actually maintains the “moral capacity” prong of the M’Naghten test because “Kansas permits a defendant to offer whatever mental health evidence he deems relevant at sentencing.”  As Justice Breyer notes in his dissent, the majority’s argument is not persuasive since “our tradition demands that an insane defendant should not be found guilty in the first place.”  Going back to the 13th century, the leading common-law jurists, such as Bracton, Coke, Hale, Blackstone, and more, agreed that a person deemed insane could not commit unlawful conduct.

Regardless of the majority’s opinion on whether the second prong of the M’Naghten test, history has shown that the idea that an insane person cannot or should not be guilty of certain crimes is an idea that most see as ingrained and fundamental to our criminal justice system.  To say this principle is preserved by allowing inquiry at sentencing undermines the historical practice of the insanity defense.

Justice Kagan also points to the Supreme Court’s ruling in Clark v. Arizona, which held that the State of Arizona could adopt a statute that included the second prong of M’Naghten (moral capacity), but not the first prong (cognitive capacity).  The question becomes, “if Arizona can eliminate the first prong of M’Naghten, why can Kansas not eliminate the second prong?”  Justice Breyer answers this question by referring back to the Court’s decision in Clark, “cognitive incapacity is itself enough to demonstrate moral incapacity.”

By allowing a defendant to offer evidence of moral incapacity, they are also allowed to offer evidence of cognitive incapacity. If a defendant cannot understand his or her actions, they certainly cannot understand the moral implications their actions carry.  In the matter of Kahler, eliminating the second prong of M’Naghten would not have the same effect that eliminating the first prong did in Clark.  Moral incapacity itself cannot show cognitive incapacity.  For example, the second prosecution (defendant believes a zombie commanded him to kill the other man) is a scenario in which the defendant had the cognitive capacity to intend to kill the other man.  However, he lacked the moral capacity to know his actions were morally wrong.  Proving that the defendant lacked the moral capacity would not tend to prove a lack of cognitive capacity.


The United States Constitution gives the States broad discretion to operate their criminal justice system, including defining crimes and defenses.  While this includes defining the complexities of the insanity defense, maintaining the foundation of this defense is fundamental to civil society.  If “the very definition of crime is conduct that merits a formal and solemn pronouncement of the moral condemnation of the community,” then the protections afforded by society are a formal and solemn declaration of their fundamental principles.  In Kahler, the Court did not merely uphold Kansas’ interpretation of a vital protection; it allowed Kansas to, in effect, eviscerate the protection entirely.  The most essential and deeply rooted values of our society should be uniformly employed to ensure the equality of laws.

Avatar photo
About Kevin Cline (3 Articles)
Kevin is a third-year student at Campbell School of Law and currently serves as an Associate Editor of the Campbell Law Observer. Hailing from the foothills of the Tar Heel State, he is from Conover, North Carolina. Prior to law school, Kevin majored in Political Science and History at Centre College in Danville, Kentucky. Beginning the summer after his first year in law school and continuing through his second year, he interns at Tarlton|Polk PLLC where he takes part in criminal defense matters at both the State and Federal level. While at Campbell Law, he is a member of The Campbell Law Softball team and is pursuing an LL.M in Legal Practice through Nottingham Trent University. He is interested in criminal law and procedure, real estate transactions, and construction law.