(Slip Opinion) OCTOBER TERM, 2019 1 

Syllabus 

NOTE: Where it is feasible, a syllabus (headnote) will be released, as is 

being done in connection with this case, at the time the opinion is issued. 

The syllabus constitutes no part of the opinion of the Court but has been 

prepared by the Reporter of Decisions for the convenience of the reader. 

See United States v. Detroit Timber & Lumber Co., 200 U. S. 321, 337. 

SUPREME COURT OF THE UNITED STATES 

Syllabus 

KAHLER v. KANSAS 

CERTIORARI TO THE SUPREME COURT OF KANSAS 

No. 18–6135. Argued October 7, 2019—Decided March 23, 2020 

In Clark v. Arizona, 548 U. S. 735, this Court catalogued the diverse 

strains of the insanity defense that States have adopted to absolve 

mentally ill defendants of criminal culpability. Two—the cognitive 

and moral incapacity tests—appear as alternative pathways to acquittal in the landmark English ruling M’Naghten’s Case, 10 Cl. & Fin. 

200, 8 Eng. Rep. 718. The moral incapacity test asks whether a defendant’s illness left him unable to distinguish right from wrong with 

respect to his criminal conduct. Respondent Kansas has adopted the 

cognitive incapacity test, which examines whether a defendant was 

able to understand what he was doing when he committed a crime. 

Specifically, under Kansas law a defendant may raise mental illness

to show that he “lacked the culpable mental state required as an element of the offense charged,” Kan. Stat. Ann §21–5209. Kansas does 

not recognize any additional way that mental illness can produce an 

acquittal, although a defendant may use evidence of mental illness to

argue for a lessened punishment at sentencing. See §§21– 

6815(c)(1)(C), 21–6625(a). In particular, Kansas does not recognize a 

moral-incapacity defense. 

Kansas charged petitioner James Kahler with capital murder after 

he shot and killed four family members. Prior to trial, he argued that

Kansas’s 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 court disagreed and the jury returned a conviction. During the penalty phase, Kahler was free to

raise any argument he wished that mental illness should mitigate his

sentence, but the jury still imposed the death penalty. The Kansas 

Supreme Court rejected Kahler’s due process argument on appeal. 

Held: Due process does not require Kansas to adopt an insanity test that

turns on a defendant’s ability to recognize that his crime was morally 
 

 

 

 

2 KAHLER v. KANSAS 

Syllabus 

wrong. Pp. 6–24.

(a) A state rule about criminal liability violates due process only if it 

“offends some principle of justice so rooted in the traditions and conscience our people as to be ranked as fundamental.” Leland v. Oregon, 

343 U. S. 790, 798 (internal quotation marks omitted). History is the 

primary guide for this analysis. The due process standard sets a high

bar, and a rule of criminal responsibility is unlikely to be sufficiently

entrenched to bind all States to a single approach. As the Court explained in Powell v. Texas, 392 U. S. 514, the scope of criminal responsibility is animated by complex and ever-changing ideas that are best

left to the States to evaluate and reevaluate over time. This principle 

applies with particular force in the context of the insanity defense, 

which also involves evolving understandings of mental illness. This 

Court has thus twice declined to constitutionalize a particular version 

of the insanity defense, see Leland, 343 U. S. 790; Clark, 548 U. S. 735, 

holding instead that a State’s “insanity rule[ ] is substantially open to

state choice,” id., at 752. Pp. 6–9.

(b) Against this backdrop, Kahler argues that Kansas has abolished

the insanity defense—and, in particular, that it has impermissibly jettisoned the moral-incapacity approach. As a starting point, Kahler is 

correct that for hundreds of years jurists and judges have recognized 

that insanity can relieve criminal responsibility. But Kansas recognizes the same: Under Kansas law, mental illness is a defense to culpability if it prevented a defendant from forming the requisite criminal

intent; a defendant is permitted to offer whatever evidence of mental

health he deems relevant at sentencing; and a judge has discretion to 

replace a defendant’s prison term with commitment to a mental health

facility.

So Kahler can prevail only by showing that due process requires 

States to adopt a specific test of insanity—namely, the moral-incapacity test. He cannot do so. Taken as a whole, the early common law

cases and commentaries reveal no settled consensus favoring Kahler’s 

preferred right-from-wrong rule. Even after M’Naghten gained popularity in the 19th century, States continued to experiment with new 

approaches. Clark therefore declared: “History shows no deference to 

M’Naghten that could elevate its formula to the level of fundamental 

principle.” 548 U. S., at 749–752. The tapestry of approaches States

have adopted shows that no single version of the insanity defense has 

become so ingrained in American law as to rank as “fundamental.” Id., 

at 749. 

This result is not surprising. Ibid. The insanity defense sits at the

juncture of medical views of mental illness and moral and legal theories of criminal culpability—two areas of conflict and change. Small 
 

 

 

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Syllabus 

wonder that no particular test of insanity has developed into a constitutional baseline. And it is not for the courts to insist on any single 

criterion moving forward. Defining the precise relationship between 

criminal culpability and mental illness requires balancing complex 

considerations, among them the workings of the brain, the purposes of 

criminal law, and the ideas of free will and responsibility. This balance 

should remain open to revision as new medical knowledge emerges and 

societal norms evolve. Thus—as the Court recognized previously in 

Leland, Powell, and Clark—the defense is a project for state governance, not constitutional law. Pp. 10–24. 

307 Kan. 374, 410 P. 3d 105, affirmed. 

KAGAN, J., delivered the opinion of the Court, in which ROBERTS, C. J., 

and THOMAS, ALITO, GORSUCH, and KAVANAUGH, JJ., joined. BREYER, J., 

filed a dissenting opinion, in which GINSBURG and SOTOMAYOR, JJ., 

joined. 
 

 

 

_________________ 

_________________ 

Cite as: 589 U. S. ____ (2020) 1 

Opinion of the Court 

NOTICE: This opinion is subject to formal revision before publication in the 

preliminary print of the United States Reports. Readers are requested to 

notify the Reporter of Decisions, Supreme Court of the United States, Washington, D. C. 20543, of any typographical or other formal errors, in order that 

corrections may be made before the preliminary print goes to press. 

SUPREME COURT OF THE UNITED STATES 

No. 18–6135 

JAMES K. KAHLER, PETITIONER v. KANSAS 

ON WRIT OF CERTIORARI TO THE SUPREME COURT OF KANSAS 

[March 23, 2020]

 JUSTICE KAGAN delivered the opinion of the Court. 

This case is about Kansas’s treatment of a criminal defendant’s insanity claim. In Kansas, a defendant can invoke mental illness to show that he lacked the requisite 

mens rea (intent) for a crime. He can also raise mental illness after conviction to justify either a reduced term of imprisonment or commitment to a mental health facility. But 

Kansas, unlike many States, will not wholly exonerate a defendant on the ground that his illness prevented him from 

recognizing his criminal act as morally wrong. The issue 

here is whether the Constitution’s Due Process Clause 

forces Kansas to do so—otherwise said, whether that 

Clause compels the acquittal of any defendant who, because 

of mental illness, could not tell right from wrong when committing his crime. We hold that the Clause imposes no such 

requirement. 

I 

A 

In Clark v. Arizona, 548 U. S. 735, 749 (2006), this Court 

catalogued state insanity defenses, counting four “strains 

variously combined to yield a diversity of American standards” for when to absolve mentally ill defendants of criminal culpability. The first strain asks about a defendant’s 
 

 

2 KAHLER v. KANSAS 

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“cognitive capacity”—whether a mental illness left him “unable to understand what he [was] doing” when he committed a crime. Id., at 747, 749. The second examines his 

“moral capacity”—whether his illness rendered him “unable to understand that his action [was] wrong.” Ibid. Those 

two inquiries, Clark explained, appeared as alternative 

pathways to acquittal in the landmark English ruling 

M’Naghten’s Case, 10 Cl. & Fin. 200, 8 Eng. Rep. 718 (H. L. 

1843), as well as in many follow-on American decisions and 

statutes: If the defendant lacks either cognitive or moral capacity, he is not criminally responsible for his behavior. Yet 

a third “building block[]” of state insanity tests, gaining 

popularity from the mid-19th century on, focuses on “volitional incapacity”—whether a defendant’s mental illness 

made him subject to “irresistible[] impulse[s]” or otherwise

unable to “control[] his actions.” Clark, 548 U. S., at 749, 

750, n. 11; see, e.g., Parsons v. State, 81 Ala. 577, 597, 2 So. 

854, 866–867 (1887). And bringing up the rear, in Clark’s 

narration, the “product-of-mental-illness test” broadly considers whether the defendant’s criminal act stemmed from 

a mental disease. 548 U. S., at 749–750. 

As Clark explained, even that taxonomy fails to capture

the field’s complexity. See id., at 750, n. 11. Most notable 

here, M’Naghten’s “moral capacity” prong later produced a 

spinoff, adopted in many States, that does not refer to morality at all. Instead of examining whether a mentally ill 

defendant could grasp that his act was immoral, some jurisdictions took to asking whether the defendant could understand that his act was illegal. Compare, e.g., People v. 

Schmidt, 216 N. Y. 324, 333–334, 110 N. E. 945, 947 (1915)

(Cardozo, J.) (asking about moral right and wrong), with, 

e.g., State v. Hamann, 285 N. W. 2d 180, 183 (Iowa 1979) 

(substituting ideas of legal right and wrong). That change

in legal standard matters when a mentally ill defendant 

knew that his act violated the law yet believed it morally 

justified. See, e.g., Schmidt, 216 N. Y., at 339, 110 N. E., at 
 

 

 

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949; People v. Serravo, 823 P. 2d 128, 135 (Colo. 1992).1 

Kansas law provides that “[i]t shall be a defense to a prosecution under any statute that the defendant, as a result of 

mental disease or defect, lacked the culpable mental state 

required as an element of the offense charged.” Kan. Stat. 

Ann. §21–5209 (2018 Cum. Supp.).2 Under that statute, a 

defendant may introduce any evidence of any mental illness

to show that he did not have the intent needed to commit 

the charged crime. Suppose, for example, that the defendant shot someone dead and goes on trial for murder. He 

may then offer psychiatric testimony that he did not understand the function of a gun or the consequences of its use—

more generally stated, “the nature and quality” of his actions. M’Naghten, 10 Cl. & Fin., at 210, 8 Eng. Rep., at 722. 

And a jury crediting that testimony must acquit him. As 

everyone here agrees, Kansas law thus uses M’Naghten’s 

“cognitive capacity” prong—the inquiry into whether a

mentally ill defendant could comprehend what he was doing when he committed a crime. See Brief for Petitioner 41; 

Brief for Respondent 31; Brief for United States as Amicus 

Curiae 18. If the defendant had no such capacity, he could 

not form the requisite intent—and thus is not criminally 

responsible.

At the same time, the Kansas statute provides that 

“[m]ental disease or defect is not otherwise a defense.” §21– 

5209. In other words, Kansas does not recognize any additional way that mental illness can produce an acquittal.3 

—————— 1Another complicating factor in Clark’s classification scheme is that 

States “limit, in varying degrees, which sorts of mental illness” can support an insanity claim. Clark v. Arizona, 548 U. S. 735, 750, n. 11 (2006).

So even two States using the same test for judging culpability may apply 

it to differently sized sets of offenders. See infra, at 21, n. 11. 2At the time of the crime in this case, a materially identical provision 

was codified at §22–3220 (2007). 3Four other States similarly exonerate a mentally ill defendant only 

when he cannot understand the nature of his actions and so cannot form 

the requisite mens rea. See Alaska Stat. §§12.47.010(a), 12.47.020 
 

4 KAHLER v. KANSAS 

Opinion of the Court 

Most important for this case, a defendant’s moral incapacity cannot exonerate him, as it would if Kansas had adopted 

both original prongs of M’Naghten. Assume, for example,

that a defendant killed someone because of an “insane delusion that God ha[d] ordained the sacrifice.” Schmidt, 216 

N. Y., at 339, 110 N. E., at 949. The defendant knew what 

he was doing (killing another person), but he could not tell

moral right from wrong; indeed, he thought the murder

morally justified. In many States, that fact would preclude 

a criminal conviction, although it would almost always lead 

to commitment in a mental health facility. In Kansas, by 

contrast, evidence of a mentally ill defendant’s moral incapacity—or indeed, of anything except his cognitive inability 

to form the needed mens rea—can play no role in determining guilt.

That partly closed-door policy changes once a verdict is

in. At the sentencing phase, a Kansas defendant has wide 

latitude to raise his mental illness as a reason to judge him 

not fully culpable and so to lessen his punishment. See 

§§21–6815(c)(1)(C), 21–6625(a). He may present evidence

(of the kind M’Naghten deemed relevant) that his disease made him unable to understand his act’s moral wrongness—as in the example just given of religious delusion. 

See §21–6625(a). Or he may try to show (in line with 

M’Naghten’s spinoff ) that the illness prevented him

from “appreciat[ing] the [conduct’s] criminality.” §21–

6625(a)(6). Or again, he may offer testimony (here invoking 

volitional incapacity) that he simply could not “conform 

[his] conduct” to legal restraints. Ibid. Kansas sentencing

law thus provides for an individualized determination of

how mental illness, in any or all of its aspects, affects culpability. And the same kind of evidence can persuade a 

court to place a defendant who needs psychiatric care in a 

—————— 

(2018); Idaho Code Ann. §§18–207(1), (3) (2016); Mont. Code Ann. §46–

14–102 (2019); Utah Code §76–2–305 (2017). 
 

 

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Opinion of the Court 

mental health facility rather than a prison. See §22–3430. 

In that way, a defendant in Kansas lacking, say, moral capacity may wind up in the same kind of institution as a like

defendant in a State that would bar his conviction. 

B 

This case arises from a terrible crime. In early 2009, Karen Kahler filed for divorce from James Kahler and moved 

out of their home with their two teenage daughters and 

9-year-old son. Over the following months, James Kahler 

became more and more distraught. On Thanksgiving weekend, he drove to the home of Karen’s grandmother, where

he knew his family was staying. Kahler entered through

the back door and saw Karen and his son. He shot Karen 

twice, while allowing his son to flee the house. He then 

moved through the residence, shooting Karen’s grandmother and each of his daughters in turn. All four of his 

victims died. Kahler surrendered to the police the next day 

and was charged with capital murder. 

Before trial, Kahler filed a motion arguing that Kansas’s 

treatment of insanity claims violates the Fourteenth

Amendment’s Due Process Clause. Kansas, he asserted, 

had “unconstitutionally abolished the insanity defense” by 

allowing the conviction of a mentally ill person “who cannot 

tell the difference between right and wrong.” App. 11–12. 

The trial court denied the motion, leaving Kahler to attempt to show through psychiatric and other testimony that

severe depression had prevented him from forming the intent to kill. See id., at 16; §21–5209. The jury convicted 

Kahler of capital murder. At the penalty phase, the court 

permitted Kahler to offer additional evidence of his mental 

illness and to argue in whatever way he liked that it should 

mitigate his sentence. The jury still decided to impose the 

death penalty.

Kahler appealed, again challenging the constitutionality 
 

 

6 KAHLER v. KANSAS 

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of Kansas’s approach to insanity claims. The Kansas Supreme Court rejected his argument, relying on an earlier 

precedential decision. See 307 Kan. 374, 400–401, 410 

P. 3d 105, 124–125 (2018) (discussing State v. Bethel, 275 

Kan. 456, 66 P. 3d 840 (2003)). There, the court denied that 

any single version of the insanity defense is so “ingrained 

in our legal system” as to count as “fundamental.” Id., at 

473, 66 P. 3d, at 851. The court thus found that “[d]ue process does not mandate that a State adopt a particular insanity test.” Ibid. 

Kahler then asked this Court to decide whether the Due 

Process Clause requires States to provide an insanity defense that acquits a defendant who could not “distinguish 

right from wrong” when committing his crime—or, otherwise put, whether that Clause requires States to adopt the

moral-incapacity test from M’Naghten. Pet. for Cert. 18. 

We granted certiorari, 586 U. S. ___ (2019), and now hold it 

does not.4 

II 

A 

A challenge like Kahler’s must surmount a high bar. Under well-settled precedent, 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.” Leland v. Oregon, 343 U. S. 790, 798 (1952) (internal quotation marks

omitted). Our primary guide in applying that standard is

“historical practice.” Montana v. Egelhoff, 518 U. S. 37, 43 

(1996) (plurality opinion). And in assessing that practice, 

—————— 4Kahler also asked us to decide whether the Eighth Amendment requires that States make available the moral-incapacity defense. See Pet. 

for Cert. 18. But that claim is not properly before us. Kahler did not 

raise the argument below, and the Kansas courts therefore did not address it. 
 

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Opinion of the Court 

we look primarily to eminent common-law authorities 

(Blackstone, Coke, Hale, and the like), as well as to early 

English and American judicial decisions. See, e.g., id., at 

44–45; Patterson v. New York, 432 U. S. 197, 202 (1977).

The question is whether a rule of criminal responsibility is

so old and venerable—so entrenched in the central values 

of our legal system—as to prevent a State from ever choosing another. An affirmative answer, though not unheard 

of, is rare. See, e.g., Clark, 548 U. S., at 752 (“[T]he conceptualization of criminal offenses” is mostly left to the States).

In Powell v. Texas, 392 U. S. 514 (1968), this Court explained why. There, Texas declined to recognize “chronic

alcoholism” as a defense to the crime of public drunkenness. 

Id., at 517 (plurality opinion). The Court upheld that decision, emphasizing the paramount role of the States in setting “standards of criminal responsibility.” Id., at 533. In 

refusing to impose “a constitutional doctrine” defining those 

standards, the Court invoked the many “interlocking and 

overlapping concepts” that the law uses to assess when a 

person should be held criminally accountable for “his antisocial deeds.” Id., at 535–536. “The doctrines of actus reus, 

mens rea, insanity, mistake, justification, and duress”—the

Court counted them off—reflect both the “evolving aims of 

the criminal law” and the “changing religious, moral, philosophical, and medical views of the nature of man.” Id., at 

536. Or said a bit differently, crafting those doctrines involves balancing and rebalancing over time complex and 

oft-competing ideas about “social policy” and “moral culpability”—about the criminal law’s “practical effectiveness” 

and its “ethical foundations.” Id., at 538, 545, 548 (Black, 

J., concurring). That “constantly shifting adjustment”

could not proceed in the face of rigid “[c]onstitution[al] formulas.” Id., at 536–537 (plurality opinion). Within broad 

limits, Powell thus concluded, “doctrine[s] of criminal responsibility” must remain “the province of the States.” Id., 

at 534, 536. 
 

 

8 KAHLER v. KANSAS 

Opinion of the Court 

Nowhere has the Court hewed more closely to that view

than in addressing the contours of the insanity defense. 

Here, uncertainties about the human mind loom large. See, 

e.g., Ake v. Oklahoma, 470 U. S. 68, 81 (1985) (“[P]sychiatrists disagree widely and frequently on what constitutes

mental illness, on [proper] diagnos[es, and] on cure and 

treatment”). Even as some puzzles get resolved, others 

emerge. And those perennial gaps in knowledge intersect 

with differing opinions about how far, and in what ways, 

mental illness should excuse criminal conduct. See Clark, 

548 U. S., at 749–752 (canvassing how those competing 

views produced a wealth of insanity tests); supra, at 1–2. 

“This whole problem,” we have noted, “has evoked wide disagreement.” Leland, 343 U. S., at 801. On such unsettled 

ground, we have hesitated to reduce “experimentation, and

freeze [the] dialogue between law and psychiatry into a 

rigid constitutional mold.” Powell, 392 U. S., at 536–537. 

Indeed, while addressing the demand for an alcoholism defense in Powell, the Court pronounced—as something close 

to self-evident—that “[n]othing could be less fruitful” than 

to define a specific “insanity test in constitutional terms.” 

Id., at 536. 

And twice before we have declined to do so. In Leland v. 

Oregon, a criminal defendant challenged as a violation of due 

process the State’s use of the moral-incapacity test of insanity—the very test Kahler now asks us to require. See 343 

U. S., at 800–801. According to the defendant, Oregon instead had to adopt the volitional-incapacity (or irresistibleimpulse) test to comply with the Constitution. See ibid.; 

supra, at 2. We rejected that argument. “[P]sychiatry,” we 

first noted, “has made tremendous strides since [the moralincapacity] test was laid down in M’Naghten’s Case,” implying that the test seemed a tad outdated. 343 U. S., at 800– 

801. But still, we reasoned, “the progress of science has not 

reached a point where its learning” would demand “eliminat[ing] the right and wrong test from [the] criminal law.” 
 

 

 

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Opinion of the Court 

Id., at 801. And anyway, we continued, the “choice of a test 

of legal sanity involves not only scientific knowledge but 

questions of basic policy” about when mental illness should 

absolve someone of “criminal responsibility.” Ibid. The 

matter was thus best left to each State to decide on its own. 

The dissent agreed (while parting from the majority on another ground): “[I]t would be indefensible to impose upon 

the States[] one test rather than another for determining 

criminal culpability” for the mentally ill, “and thereby to 

displace a State’s own choice.” Id., at 803 (opinion of Frankfurter, J.).

A half-century later, we reasoned similarly in Clark. 

There, the defendant objected to Arizona’s decision to discard the cognitive-incapacity prong of M’Naghten and leave 

in place only the moral-incapacity one—essentially the flipside of what Kansas has done. Again, we saw no due process problem. Many States, we acknowledged, allowed a 

defendant to show insanity through either prong of 

M’Naghten. See 548 U. S., at 750. But we denied that this 

approach “represents the minimum that a government 

must provide.” Id., at 748. In so doing, we invoked the

States’ traditional “capacity to define crimes and defenses,” 

and noted how views of mental illness had been particularly

“subject to flux and disagreement.” Id., at 749, 752. And 

then we surveyed the disparate ways that state laws had 

historically excused criminal conduct because of mental disease—those “strains variously combined to yield a diversity

of American standards.” See id., at 749–752; supra, at 1–2. 

The takeaway was “clear”: A State’s “insanity rule[] is substantially open to state choice.” Clark, 548 U. S., at 752. 

Reiterating Powell’s statement, Clark held that “no particular” insanity test serves as “a baseline for due process.” 

548 U. S., at 752. Or said just a bit differently, that “due

process imposes no single canonical formulation of legal insanity.” Id., at 753. 
 

10 KAHLER v. KANSAS 

Opinion of the Court 

B 

Yet Kahler maintains that Kansas’s treatment of insanity fails to satisfy due process. He sometimes makes his 

argument in the broadest of strokes, as he did before trial. 

See supra, at 5. Kansas, he then contends, has altogether 

“abolished the insanity defense,” in disregard of hundreds

of years of historical practice. Brief for Petitioner 39. His 

central claim, though, is more confined. It is that Kansas 

has impermissibly jettisoned the moral-incapacity test for

insanity. See id., at 12, 23. As earlier noted, both Clark 

and Leland described that test as coming from M’Naghten. 

See 548 U. S., at 749; 343 U. S., at 801; supra, at 2, 8. But 

according to Kahler (and the dissent), the moral-incapacity

inquiry emerged centuries before that decision, thus forming part of the English common-law heritage this country

inherited. See Brief for Petitioner 21, 42; post, at 4–14 

(opinion of BREYER, J.). And the test, he claims, served for 

all that time—and continuing into the present—as the

touchstone of legal insanity: If a defendant could not understand that his act was morally wrong, then he could not be

found criminally liable. See Brief for Petitioner 20–23; 

see also post, at 15. So Kahler concludes that the moralincapacity standard is a “principle of justice so rooted in the 

traditions and conscience of our people as to be ranked as

fundamental.” Leland, 343 U. S., at 798; see supra, at 6. In 

essence—and contra Clark—that test is the “single canonical formulation of legal insanity” and thus the irreducible 

“baseline for due process.” 548 U. S., at 752–753; see supra, 

at 9.5 

—————— 5Although the dissent at times claims to the contrary, its argument is 

the same. Given the clear direction of our precedent, the dissent must 

purport to grant the States “leeway” in defining legal insanity. Post, at 

1. But the entirety of the dissent’s historical analysis focuses on the 

moral-incapacity standard—attempting to show, just as Kahler does, 

that it both preceded and succeeded M’Naghten. See post, at 4–17. And 

in line with that narration, the dissent insists on moral understanding 
Cite as: 589 U. S. ____ (2020) 11 

Opinion of the Court 

One point, first, of agreement: Kahler is right that for

hundreds of years jurists and judges have recognized insanity (however defined) as relieving responsibility for a crime. 

“In criminal cases therefore,” Sir William Blackstone wrote, 

“lunatics are not chargeable for their own acts, if committed 

when under these incapacities.” 4 Commentaries on the 

Laws of England 24 (1769). Sir Edward Coke even earlier 

explained that in criminal cases, “the act and wrong of a 

mad man shall not be imputed to him.” 2 Institutes of the 

Laws of England §405, p. 247b (1628) (Coke). And so too 

Henry de Bracton thought that a “madman” could no sooner

be found criminally liable than a child. 2 Bracton on Laws 

and Customs of England 384 (S. Thorne transl. 1968) (Bracton). That principle of non-culpability appeared in case after case involving allegedly insane defendants, on both 

sides of the Atlantic. “The defense of insanity[] is a defense 

for all crimes[,] from the highest to the lowest,” said the

Court in Old Bailey. Trial of Samuel Burt (July 19, 1786), 

in 6 Proceedings in the Old Bailey 874 (E. Hodgson ed.

1788) (Old Bailey Proceedings). Repeated Justice Story,

when riding circuit: “In general, insanity is an excuse for 

the commission of every crime, because the party has not

the possession of that reason, which includes responsibility.” United States v. Drew, 25 F. Cas. 913 (No. 14,993) (CC 

Mass. 1828); see also, e.g., State v. Marler, 2 Ala. 43, 49 

(1841) (“If the prisoner was insane, he was not an accountable being”); Cornwell v. State, 8 Tenn. 147, 156 (1827) 

(“[P]erfect madness” will “free a man from punishment for 

crime”). We have not found a single case to the contrary. 

—————— 

as the indispensable criterion of legal sanity—the sine qua non of criminal responsibility. See, e.g., post, at 1, 3–4, 8–9, 18–21. Indeed, the dissent offers only one way the States have actual “leeway” to change their

insanity rules: They can “expand upon M’Naghten’s principles” by finding that even some who have moral capacity are insane. Post, at 22. But 

that is just to say that moral capacity is the constitutional floor—again, 

exactly what Kahler argues. 
 

 

12 KAHLER v. KANSAS 

Opinion of the Court 

But neither do we think Kansas departs from that broad 

principle. First, Kansas has an insanity defense negating 

criminal liability—even though not the type Kahler demands. As noted earlier, Kansas law provides that it is “a 

defense to a prosecution” that “the defendant, as a result of 

mental disease or defect, lacked the culpable mental state 

required” for a crime. §21–5209; see supra, at 3. That provision enables a defendant to present psychiatric and other 

evidence of mental illness to defend himself against a criminal charge. More specifically, the defendant can use that

evidence to show that his illness left him without the cognitive capacity to form the requisite intent. See supra, at 3. 

Recall that such a defense was exactly what the defendant 

in Clark wanted, in preference to Arizona’s moral-incapacity 

defense: His (unsuccessful) appeal rested on the trial court’s

exclusion of psychiatric testimony to show that he lacked 

the relevant mens rea. See 548 U. S., at 745–747; supra, at 

9. Here, Kahler could do what Clark could not—try to show

through such testimony that he had no intent to kill. Of 

course, Kahler would have preferred Arizona’s kind of insanity defense (just as Clark would have liked Kansas’s). 

But that does not mean that Kansas (any more than Arizona) failed to offer any insanity defense at all.

Second, and significantly, Kansas permits a defendant to

offer whatever mental health evidence he deems relevant 

at sentencing. See §§21–6815(c)(1)(C), 21–6625(a); supra, 

at 4. A mentally ill defendant may argue there that he is

not blameworthy because he could not tell the difference between right and wrong. Or, because he did not know his 

conduct broke the law. Or, because he could not control his 

behavior. Or, because of anything else. In other words, any 

manifestation of mental illness that Kansas’s guilt-phase 

insanity defense disregards—including the moral incapacity Kahler highlights—can come in later to mitigate culpability and lessen punishment. And that same kind of evidence can persuade a judge to replace any prison term with 
 

Cite as: 589 U. S. ____ (2020) 13 

Opinion of the Court 

commitment to a mental health facility. See §22–3430; supra, at 4–5. So as noted above, a defendant arguing moral 

incapacity may well receive the same treatment in Kansas 

as in States that would acquit—and, almost certainly, commit—him for that reason. See supra, at 4–5. In sum, Kansas does not bar, but only channels to sentencing, the mental health evidence that falls outside its intent-based 

insanity defense. When combined with Kansas’s allowance 

of mental health evidence to show a defendant’s inability to

form criminal intent, that sentencing regime defeats

Kahler’s charge that the State has “abolish[ed] the insanity 

defense entirely.”6 Brief for Petitioner 39. 

So Kahler can prevail here only if he can show (again, 

contra Clark) that due process demands a specific test of

legal insanity—namely, whether mental illness prevented 

a defendant from understanding his act as immoral. Kansas, as we have explained, does not use that type of insanity

rule. See supra, at 3–4. If a mentally ill defendant had 

enough cognitive function to form the intent to kill, Kansas

law directs a conviction even if he believed the murder morally justified. In Kansas’s judgment, that delusion does not 

make an intentional killer entirely blameless. See Brief for 

Respondent 40. Rather than eliminate, it only lessens the

defendant’s moral culpability. See ibid. And sentencing is

the appropriate place to consider mitigation: The decisionmaker there can make a nuanced evaluation of blame, 

rather than choose, as a trial jury must, between all and 

nothing. See ibid. In any event, so Kansas thinks.7 Those 

—————— 6We here conclude only that Kansas’s scheme does not abolish the insanity defense. We say nothing, one way or the other, about whether any

other scheme might do so. 7The dissent is therefore wrong to suggest that Kansas’s law has become untethered from moral judgments about culpability. See post, at 1, 

3, 16–22. No doubt, Kansas’s moral judgments differ from the dissent’s. 

Again, Kansas believes that an intentional killer is not wholly blameless, 

even if, for example, he thought his actions commanded by God. The 

dissent, in contrast, considers Kansas’s view benighted (as maybe some 
 

 

 

 

14 KAHLER v. KANSAS 

Opinion of the Court 

views are contested and contestable; other States—many 

others—have made a different choice. But Kahler must 

show more than that. He must show that adopting the 

moral-incapacity version of the insanity rule is not a choice 

at all—because, again, that version is “so rooted in the 

traditions and conscience of our people as to be ranked as

fundamental.” Leland, 343 U. S., at 798. And he cannot. 

The historical record is, on any fair reading, complex—even 

messy. As we will detail, it reveals early versions of

not only Kahler’s proposed standard but also Kansas’s

alternative. 

Early commentators on the common law proposed various formulations of the insanity defense, with some favoring a morality inquiry and others a mens rea approach.

Kahler cites William Lambard’s 16th-century treatise defining a “mad man” as one who “hath no knowledge of good 

nor evil” (the right and wrong of the day). Eirenarcha, ch. 

21, p. 218 (1581). He likewise points to William Hawkins’s

statement, over a hundred years later, that a “lunatick[]” is

not punishable because “under a natural disability of distinguishing between good and evil.” 1 Pleas of the Crown 

§1, p. 2 (1716) (capitalization omitted). Both true enough. 

But other early versions of the insanity test—and from a 

more famous trio of jurists—demanded the kind of cognitive 

—————— 

in the majority do too). But that is not a dispute, as the dissent suggests, 

about whether morality should play a role in assigning legal responsibility. It is instead a disagreement about what morality entails—that is, 

about when a defendant is morally culpable for an act like murder. See 

State v. Bethel, 275 Kan. 456, 465–471, 66 P. 3d 840, 847–850 (2003) (accepting Kansas’s view that “moral blameworthiness” is linked to a defendant’s intent to kill, rather than to his ability to tell right from wrong). 

And we have made clear, from Leland to Powell to Clark, that courts do 

not get to make such judgments. See supra, at 7–9. Instead, the States 

have broad discretion to decide who counts as blameworthy, and to weigh

that along with other factors in defining the elements of, and defenses 

to, crimes. 
 

 

 

Cite as: 589 U. S. ____ (2020) 15 

Opinion of the Court 

impairment that prevented a defendant from understanding the nature of his acts, and thus intending his crime. 

Henry de Bracton’s 13th-century treatise gave rise to what

became known as the “wild beast” test. See J. Biggs, The 

Guilty Mind 82 (1955). Used for hundreds of years, it likened a “madman” to an “animal[] which lack[s] reason” and 

so could not have “the intention to injure.” Bracton 384; see 

ibid. (A “madman” cannot commit a crime because “[i]t is

will and purpose which mark” misdeeds). Sir Edward Coke 

similarly linked the definition of insanity to a defendant’s 

inability to form criminal intent. He described a legally insane person in 1628 as so utterly “without his mind or discretion” that he could not have the needed mens rea. 2 Coke 

§405, at 247b. So too Lord Matthew Hale a century later.

He explained that insanity involves “a total alienation of 

the mind or perfect madness,” such that a defendant could

not act “animo felonico,” meaning with felonious intent. 1 

Pleas of the Crown, ch. 4, pp. 30, 37 (1736); see id., at 37 

(“[F]or being under a full alienation of mind, he acts not per 

electionem or intentionem [by choice or intent]”).8 

—————— 8The dissent tries to recruit these three jurists to the side of the moralincapacity test, see post, at 5–7, but cannot succeed. Even the carefully 

curated passages the dissent quotes focus on cognitive capability rather 

than moral judgment. See, e.g., post, at 5–6 (asking whether a defendant 

had “sense and reason” or “understanding and liberty of will”). In so 

doing, they refer to the defendant’s ability to form the requisite mens rea, 

or felonious intent. See Clark, 548 U. S., at 747; supra, at 1–3. 

The dissent still insists all is not lost because (it says) mens rea itself 

hinged at common law on a defendant’s “moral understanding.” Post, at 

8–9. Here, the dissent infers from the use of “good-from-evil” language

in various common-law treatises and cases that moral blameworthiness 

must have defined the mens rea inquiry. See ibid. But to begin with—

and to repeat the point made in the text—the most influential treatises

used little of that language, emphasizing instead the need for a defendant to intend his act in the ordinary sense of the term. And as we will 

explain, the joint presence of references to mens rea and moral understanding in other common-law sources involving insanity does not show

that most jurists saw the two concepts as one and the same. See infra, 
 

16 KAHLER v. KANSAS 

Opinion of the Court 

Quite a few of the old common-law cases similarly

stressed the issue of cognitive capacity. To be sure, even 

these cases included some references to the ability to tell 

right from wrong (and the dissent eagerly cherry-picks

every one of them). But the decisions’ overall focus was less 

on whether a defendant thought his act moral than on

whether he had the ability to do much thinking at all. In 

the canonical case of Rex v. Arnold, 16 How. St. Tr. 695 

(1724), for example, the jury charge descended straight

from Bracton: 

“[I]t is not every kind of frantic humour or something 

unaccountable in a man’s actions, that points him out

to be such a madman as is to be exempted from punishment: it must be a man that is totally deprived of his

understanding and memory, and doth not know what 

he is doing, no more than an infant, than a brute, or a 

wild beast.” Id., at 764–765. 

And the court offered an accompanying test linking that

lack of reason to mens rea: If a man is “deprived of his reason, and consequently of his intention, he cannot be guilty.” 

Id., at 764; see ibid. (defining a “madman” as a “person that 

hath no design”); see also Trial of William Walker (Apr. 21,

1784), in 4 Old Bailey Proceedings 544, 547 (asking whether 

the defendant had a “distemper of mind which had deprived

him of the use of his reason” or instead whether “he knew 

what he was doing [and] meant to do it”); Beverley’s Case, 4 

Co. Rep. 123b, 124b, 76 Eng. Rep. 1118, 1121 (K. B. 1603) 

—————— 

at 16–19. Some may well have viewed mens rea through a moral prism;

but others emphasized cognitive understanding in using that term; and

still others combined the moral and cognitive in diverse ways. Which is 

to say that the record is far more complicated than the dissent lets on, 

with jurists invoking, both within particular sources and across all of 

them, a variety of ways to resolve insanity claims. And under our longestablished precedent, that motley sort of history cannot provide the basis for a successful due process claim. 
 

 

Cite as: 589 U. S. ____ (2020) 17 

Opinion of the Court 

(asking whether a man “is deprived of reason and understanding” and so “cannot have a felonious intent”). The 

House of Lords used much the same standard in Rex v. Lord 

Ferrers, 19 How. St. Tr. 886 (1760), when sitting in judgment on one of its members. There, the Solicitor General 

told the Lords to address “the capacity and intention of the 

noble prisoner.” Id., at 948. Relying heavily on Hale’s treatise, he defined the legally insane as suffering from an “alienation of mind” and a “total[] want of reason.” Id., at 947. 

And in recapping the evidence on that issue, he asked about 

the defendant’s intention: “Did [Ferrers] proceed with deliberation? Did he know the consequences” of his act? Id., 

at 948.9 

In such cases, even the language of morality mostly 

worked in service of the emphasis on cognition and mens 

rea. The idea was that if a defendant had such a “total[] 

want of reason” as to preclude moral thinking, he could not 

possibly have formed the needed criminal intent. Id., at 

947. Lord Chief Justice Mansfield put the point neatly in 

Bellingham’s Case, 1 G. Collinson, Treatise on the Law Concerning Idiots, Lunatics, and Other Persons Non Compotes 

—————— 9Even in the face of these instructions, the dissent claims that Arnold 

and Ferrers actually used the moral-incapacity test. See post, at 9–11. 

The assertion is based on some “good and evil” language (in Ferrers, 

mostly from witnesses) appearing in the case reports. But scholars generally agree, in line with our view, that Arnold and Ferrers “demonstrate 

how strictly” courts viewed “the criteria of insanity.” 1 N. Walker, Crime 

and Insanity in England 53 (1968) (noting that the two decisions “have 

often been cited” for that proposition). Kahler himself does not dispute 

the point; indeed, he essentially concedes our reading. Rather than try

to make the decisions say something they do not, he argues only that 

they were “outlier[s]” and “could hardly have been less typical.” Brief for 

Petitioner 22, n. 5; Reply Brief 4 (internal quotation marks omitted). But 

that contrasting response fares no better. As even the dissent agrees,

these were the “seminal” common-law decisions relating to insanity—indeed, two of only a small number in that period to make it into official 

reports. Post, at 9. 
 

 

18 KAHLER v. KANSAS 

Opinion of the Court 

Mentis 636 (1812) (Collinson). He instructed the jury: 

“If a man were deprived of all power of reasoning, so as

not to be able to distinguish whether it was right or

wrong to commit the most wicked transaction, he could

not certainly do an act against the law. Such a man, so 

destitute of all power of judgment, could have no intention at all.” Id., at 671. 

On that account, moral incapacity was a byproduct of the 

kind of cognitive breakdown that precluded finding mens 

rea, rather than a self-sufficient test of insanity. See also 

Rex v. Offord, 5 Car. & P. 168, 169, 172 Eng. Rep. 924, 925 

(N. P. 1831) (“express[ing] complete accordance in the observations of th[e] learned Judge” in Bellingham). Or said 

another way, a mentally ill defendant’s inability to distinguish right from wrong, rather than independently producing an insanity acquittal, served as a sign—almost a kind 

of evidence—that the defendant lacked the needed criminal 

intent. 

Other early common-law cases do not adopt the mens rea 

approach—but neither can they sustain Kahler’s position.

Kahler relies mainly on Hadfield’s Case, 27 How. St. Tr. 

1281 (1800), to show that common-law courts would acquit 

a mentally ill defendant who understood the nature of his 

act, but believed it moral. See Reply Brief 4. There, the 

defendant had deliberately set out to assassinate King

George III on the view that doing so would bring about the 

Second Coming. See 27 How. St. Tr., at 1322. The judge

instructed the jury that the defendant was so “deranged” as

to make acquittal appropriate. Id., at 1353. Maybe, as 

Kahler argues, that directive stemmed from the defendant’s

inability to tell right from wrong. But the judge never used

that language, or stated any particular legal standard, so it

is hard to know. Still other judges explained insanity to

juries by throwing everything against the wall—mixing notions of cognitive incapacity, moral incapacity, and more, 
Cite as: 589 U. S. ____ (2020) 19 

Opinion of the Court 

without trying to order, prioritize, or even distinguish

among them. See, e.g., Regina v. Oxford, 9 Car. & P. 525, 

545–548, 173 Eng. Rep. 941, 950 (N. P. 1840); Trial of Francis Parr (Jan. 15, 1787), in 2 Old Bailey Proceedings 228– 

229; Bowler’s Case, 1 Collinson 674. Those decisions treat 

the inability to make moral judgments more as part of an 

all-things-considered assessment of legal insanity, and less 

as its very definition. But even if some of them belong in

Kahler’s corner, that would be far from enough. Taken as 

a whole, the common-law cases reveal no settled consensus 

favoring Kahler’s preferred insanity rule. And without 

that, they cannot support his proposed constitutional

baseline.

 Only with M’Naghten, in 1843, did a court articulate, and 

momentum grow toward accepting, an insanity defense 

based independently on moral incapacity. See Clark, 548 

U. S., at 749; Leland, 343 U. S., at 801; supra, at 2, 8. The 

M’Naghten test, as already described, found insanity in either of two circumstances. See supra, at 1–2. A defendant 

was acquitted if he “labour[ed] under such a defect of reason, from disease of the mind, [1] as not to know the nature 

and quality of the act he was doing; or, [2] if he did know it, 

that he did not know he was doing what was wrong.” 10 Cl. 

& Fin., at 210, 8 Eng. Rep., at 722 (emphasis added). That 

test disaggregated the concepts of cognitive and moral incapacity, so that each served as a stand-alone defense. And 

its crisp two-part formulation proved influential, not only

in Great Britain but in the United States too. Over the 

course of the 19th century, many States adopted the test, 

making it the most popular one in the country. 

Still, Clark unhesitatingly declared: “History shows no

deference to M’Naghten that could elevate its formula to the 

level of fundamental principle.” 548 U. S., at 749. As Clark 

elaborated, even M’Naghten failed to unify state insanity

defenses. See 548 U. S., at 749–752. States continued to 

experiment with insanity rules, reflecting what one court 
 

 

 

 

 

 

20 KAHLER v. KANSAS 

Opinion of the Court 

called “the infinite variety of forms [of] insanity” and the 

“difficult and perplexing” nature of the defense. Roberts v. 

State, 3 Ga. 310, 328, 332 (1847). Some States in the 1800s 

gravitated to the newly emergent “volitional incapacity” 

standard, focusing on whether the defendant could at all 

control his actions. Clark, 548 U. S., at 749; see, e.g., Roberts, 3 Ga., at 331. One court viewed that inquiry as “much

more practical” than the “right and wrong test,” which it 

thought often “speculative and difficult of determination.” 

State v. Felter, 25 Iowa 67, 82, 84 (1868); see Leland, 343 

U. S., at 801 (recognizing such skepticism about the moralincapacity test); supra, at 8–9. Another prophesied that the

volitional test was the one “towards which all the modern 

authorities in this country[] are gradually but surely tending.” Parsons, 81 Ala., at 586, 2 So., at 859. But that test, 

too, failed to sweep all before it: State innovation proceeded 

apace. See, e.g., State v. Pike, 49 N. H. 399, 442 (1870) (applying the “product” test, which excuses a defendant whose

crime “was the offspring or product of mental disease”);

N. D. Cent. Code Ann. §12.1–04.1–01(1)(a) (2012) (replacing the right-from-wrong test with an inquiry into whether 

the defendant’s act arose from “[a] serious distortion of 

[his] capacity to recognize reality”). Much as medical views 

of mental illness changed as time passed, so too did legal 

views of how to account for that illness when assigning 

blame. 

As earlier noted, even the States that adopted M’Naghten soon divided on what its second prong should mean. 

See supra, at 2–3. Most began by asking, as Kahler does,

about a defendant’s ability to grasp that his act was immoral. See, e.g., Wright v. State, 4 Neb. 407, 409 (1876); 

State v. Spencer, 21 N. J. L. 196, 201 (1846). Thus, Clark 

labeled M’Naghten’s second prong a test of “moral capacity,” and invoked the oft-used phrase “telling right from 

wrong” (or in older language, good from evil) to describe its

central inquiry. 548 U. S., at 747, 753; see supra, at 2. But 
 

 

 

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Opinion of the Court 

over the years, 16 States have reoriented the test to focus

on the defendant’s understanding that his act was illegal— 

that is, legally rather than morally “wrong.”10 They thereby

excluded from the ranks of the insane those who knew an 

act was criminal but still thought it right. 

Contrary to Kahler’s (and the dissent’s) contention, that

difference matters. See Reply Brief 7 (claiming that “there 

is little daylight between these inquiries”); post, at 17, 21 

(same). The two tests will treat some, even though not all,

defendants in opposite ways. And the defendants they will

treat differently are exactly those Kahler (and the dissent) 

focus on: those who know exactly what they are doing (including that it is against the law) but believe it morally justified—because, say, it is commanded by God (or in the dissent’s case, a dog). See Brief for Petitioner 15; post, at 20; 

Schmidt, 216 N. Y., at 339, 110 N. E., at 949.11 A famed 

—————— 10See State v. Skaggs, 120 Ariz. 467, 472, 586 P. 2d 1279, 1284 (1978); 

Wallace v. State, 766 So. 2d 364, 367 (Fla. App. 2000); State v. Hamann, 

285 N. W. 2d 180, 184 (Iowa 1979); Commonwealth v. Lawson, 475 Mass. 

806, 811, 62 N. E. 3d 22, 28 (2016); State v. Worlock, 117 N. J. 596, 610– 

611, 569 A. 2d 1314, 1322 (1990); People v. Wood, 12 N. Y. 2d 69, 76, 187 

N. E. 2d 116, 121–122 (1962); State v. Carreiro, 2013–Ohio–1103, 988 

N. E. 2d 21, 27 (App.); McElroy v. State, 242 S. W. 883, 884 (Tenn. 1922); 

McAfee v. State, 467 S. W. 3d 622, 636 (Tex. Crim. App. 2015); State v. 

Crenshaw, 98 Wash. 2d 789, 794–795, 659 P. 2d 488, 492–493 (1983); 

Ark. Code Ann. §5–2–301(6) (2017); Ill. Comp. Stat., ch. 720, §5/6–2(a)

(West 2016); Ky. Rev. Stat. Ann. §504.020(1) (West 2016); Md. Crim. 

Proc. Code Ann. §3–109(a) (2018); Ore. Rev. Stat. §161.295(1) (2019); Vt. 

Stat. Ann., Tit. 13, §4801(a)(1) (2019). 11The great judge (later Justice) whom the dissent cites to suggest

there is no real difference between the legal wrong and moral wrong tests 

wrote a lengthy opinion whose point was the opposite. Consider a case, 

Judge Cardozo said: “A mother kills her infant child to whom she has 

been devotedly attached. She knows the nature and quality of the act;

she knows that the law condemns it; but she is inspired by an insane 

delusion that God has appeared to her and ordained the sacrifice.” People v. Schmidt, 216 N. Y. 324, 339, 110 N. E. 945, 949 (1915). If the legal 

wrong test were used, Judge Cardozo continued, “it would be the duty of 

a jury to hold her responsible for the crime.” Ibid. But not if the focus 
 

 

 

 

22 KAHLER v. KANSAS 

Opinion of the Court 

theorist of criminal law put the point this way: 

“A kills B knowing that he is killing B, and knowing 

that it is illegal to kill B, but under an insane delusion 

that the salvation of the human race will be obtained 

by . . . the murder of B[.] A’s act is a crime if the word 

‘wrong’ [in M’Naghten] means illegal. It is not a crime 

if the word wrong means morally wrong.” 2 J. Stephen,

History of the Criminal Law of England, ch. 19, p. 149 

(1883). 

So constitutionalizing the moral-incapacity standard, as

Kahler requests, would require striking down not only the 

five state laws like Kansas’s (as the dissent at times suggests, see post, at 16), but 16 others as well (as the dissent 

eventually concedes is at least possible, see post, at 21).

And with what justification? The emergence of M’Naghten’s legal variant, far from raising a due process problem,

merely confirms what Clark already recognized. Even after 

its articulation in M’Naghten (much less before), the moralincapacity test has never commanded the day. Clark, 548 

U. S., at 749.12 

—————— 

was, as in the original M’Naghten test, on moral wrong. And that difference led the New York Court of Appeals to hold that the trial court’s jury

instruction was in error. See 216 N. Y., at 340, 110 N. E., at 950. The 

additional cases the dissent cites to downplay the distinction between 

moral and legal wrong in fact follow Schmidt in recognizing when they 

diverge. See Worlock, 117 N. J., at 611, 569 A. 2d, at 1322 (explaining 

that “the distinction between moral and legal wrong may be critical” 

when, for example, a defendant “knowingly kill[s] another in obedience 

to a command from God”); Crenshaw, 98 Wash. 2d, at 798, 659 P. 2d, at 

494 (acknowledging Schmidt’s view that even when a defendant “knows 

that the law and society condemn [her] act,” she should not be held responsible if “her free will has been subsumed by her belief in [a] deific

decree”). 12The diversity of American approaches to insanity is also evident in 

the States’ decisions about which kinds of mental illness can support the 

defense. See Clark, 548 U. S., at 750, n. 11; supra, at 3, n. 1. Some States 

limit the defense to those with a “severe” mental disease. See, e.g., Ala. 
Cite as: 589 U. S. ____ (2020) 23 

Opinion of the Court 

Indeed, just decades ago Congress gave serious consideration to adopting a mens rea approach like Kansas’s as the 

federal insanity rule. See United States v. Pohlot, 827 F. 2d 

889, 899, and n. 9 (CA3 1987) (describing bipartisan support for that proposal). The Department of Justice at the

time favored that version of the insanity test. Perhaps

more surprisingly, the American Medical Association did

too. And the American Psychiatric Association took no position one way or the other. Although Congress chose in the 

end to adhere to the M’Naghten rule, the debate over the 

bill itself reveals continuing division over the proper scope 

of the insanity defense. 

Nor is that surprising, given the nature of the inquiry. As 

the American Psychiatric Association once noted, “insanity 

is a matter of some uncertainty.” Insanity Defense Work 

Group, Statement on the Insanity Defense, 140 Am. J. 

Psych. 681, 685 (1983). Across both time and place, doctors 

and scientists have held many competing ideas about mental illness. And that is only the half of it. Formulating an

insanity defense also involves choosing among theories of

moral and legal culpability, themselves the subject of recurrent controversy. At the juncture between those two 

spheres of conflict and change, small wonder there has not 

—————— 

Code §13A–3–1 (2015). Others prohibit its assertion by defendants with

specific mental disorders. See, e.g., Ariz. Rev. Stat. Ann. §13–502 (2010) 

(“psychosexual” or “impulse control disorders”); Ore. Rev. Stat. 

§161.295(2) (“personality disorders”). In particular, many States follow

the Model Penal Code in prohibiting psychopaths from raising the defense. See ALI, Model Penal Code §4.01(2), p. 163 (1985); e.g., Ind. Code 

§35–41–3–6(b) (2019) (“abnormality manifested only by repeated unlawful or otherwise antisocial conduct”). All those limitations apply even

when the defendant’s mental illness prevented him from recognizing 

that his crime was immoral. In that way too, many States have departed

from the principle that Kahler (along with the dissent) claims the Constitution commands. 
 

24 KAHLER v. KANSAS 

Opinion of the Court 

been the stasis Kahler sees—with one version of the insanity defense entrenched for hundreds of years.

And it is not for the courts to insist on any single criterion

going forward. We have made the point before, in Leland, 

Powell, and Clark. See supra, at 7–9. Just a brief reminder: 

“[F]ormulating a constitutional rule would reduce, if not 

eliminate, [the States’] fruitful experimentation, and freeze 

the developing productive dialogue between law and psychiatry into a rigid constitutional mold.” Powell, 392 U. S., 

at 536–537. Or again: In a sphere of “flux and disagreement,” with “fodder for reasonable debate about what the 

cognate legal and medical tests should be,” due process imposes no one view of legal insanity. Clark, 548 U. S., at 

752–753. Defining the precise relationship between criminal culpability and mental illness involves examining the 

workings of the brain, the purposes of the criminal law, the 

ideas of free will and responsibility. It is a project demanding hard choices among values, in a context replete with

uncertainty, even at a single moment in time. And it is a 

project, if any is, that should be open to revision over time, 

as new medical knowledge emerges and as legal and moral

norms evolve. Which is all to say that it is a project for state

governance, not constitutional law. 

We therefore decline to require that Kansas adopt an insanity test turning on a defendant’s ability to recognize that

his crime was morally wrong. Contrary to Kahler’s view, 

Kansas takes account of mental health at both trial and 

sentencing. It has just not adopted the particular insanity 

defense Kahler would like. That choice is for Kansas to 

make—and, if it wishes, to remake and remake again as the 

future unfolds. No insanity rule in this country’s heritage

or history was ever so settled as to tie a State’s hands centuries later. For that reason, we affirm the judgment below. 

It is so ordered. 
 

 

_________________ 

_________________ 

Cite as: 589 U. S. ____ (2020) 1 

BREYER, J., dissenting 

SUPREME COURT OF THE UNITED STATES 

No. 18–6135 

JAMES K. KAHLER, PETITIONER v. KANSAS 

ON WRIT OF CERTIORARI TO THE SUPREME COURT OF KANSAS 

[March 23, 2020]

 JUSTICE BREYER, with whom JUSTICE GINSBURG and 

JUSTICE SOTOMAYOR join, dissenting. 

Like the Court, I believe that the Constitution gives the

States broad leeway to define state crimes and criminal procedures, including leeway to provide different definitions

and standards related to the defense of insanity. But here, 

Kansas has not simply redefined the insanity defense.

Rather, it has eliminated the core of a defense that has 

existed for centuries: that the defendant, due to mental illness, lacked the mental capacity necessary for his conduct to be

considered morally blameworthy. Seven hundred years of

Anglo-American legal history, together with basic principles long inherent in the nature of the criminal law itself,

convince me that Kansas’ law “‘offends . . . principle[s] of 

justice so rooted in the traditions and conscience of our people as to be ranked as fundamental.’” Leland v. Oregon, 

343 U. S. 790, 798 (1952) (quoting Snyder v. Massachusetts, 

291 U. S. 97, 105 (1934)). 

I 

A much-simplified example will help the reader understand the conceptual distinction that is central to this case. 

Consider two similar prosecutions for murder. In Prosecution One, the accused person has shot and killed another 

person. The evidence at trial proves that, as a result of severe mental illness, he thought the victim was a dog. Prosecution Two is similar but for one thing: The evidence at 
 

2 KAHLER v. KANSAS 

BREYER, J., dissenting 

trial proves that, as a result of severe mental illness, the

defendant thought that a dog ordered him to kill the victim. 

Under the insanity defense as traditionally understood, the 

government cannot convict either defendant. Under Kansas’ rule, it can convict the second but not the first. 

To put the matter in more explicitly legal terms, consider 

the most famous statement of the traditional insanity defense, that contained in M’Naghten’s Case, 10 Cl. & Fin. 

200, 8 Eng. Rep. 718 (H. L. 1843). Lord Chief Justice Tindal, speaking for a majority of the judges of the commonlaw courts, described the insanity defense as follows: 

“[T]o establish a defence on the ground of insanity, it 

must be clearly proved that, at the time of the committing of the act, the party accused was labouring under

such a defect of reason, from disease of the mind, [1] as

not to know the nature and quality of the act he was

doing; or, [2] if he did know it, that he did not know he 

was doing what was wrong.” Id., at 210, 8 Eng. Rep.,

at 722. 

The first prong (sometimes referred to as “cognitive incapacity”) asks whether the defendant knew what he was doing. This prong corresponds roughly to the modern concept 

of mens rea for many offenses. The second (sometimes referred to as “moral incapacity”) goes further. It asks, even 

if the defendant knew what he was doing, did he have the 

capacity to know that it was wrong? Applying this test to

my example, a court would find that both defendants successfully established an insanity defense. Prosecution One 

(he thought the victim was a dog) falls within M’Naghten’s 

first prong, while Prosecution Two (he thought the dog ordered him to do it) falls within its second prong. 

In Kansas’ early years of statehood, its courts recognized

the M’Naghten test as the “cardinal rule of responsibility in 

the criminal law.” State v. Nixon, 32 Kan. 205, 206, 4 

P. 159, 160 (1884). Kansas “steadfastly adhered to that 
 

Cite as: 589 U. S. ____ (2020) 3 

BREYER, J., dissenting 

test” for more than a century. State v. Baker, 249 Kan. 431, 

449–450, 819 P. 2d 1173, 1187 (1991). But in 1995, Kansas 

“‘legislatively abolish[ed] the insanity defense.’” State v. 

Jorrick, 269 Kan. 72, 82, 4 P. 3d 610, 617 (2000) (quoting 

Rosen, Insanity Denied: Abolition of the Insanity Defense

in Kansas, 8 Kan. J. L. & Pub. Pol’y 253, 254–255 (1997)).

Under the new provision, a criminal defendant’s mental 

disease or defect is relevant to his guilt or innocence only 

insofar as it shows that he lacked the intent defined as an 

element of the offense, or mens rea. If the defendant acted 

with the required level of intent, then he has no defense 

based on mental illness. Kan. Stat. Ann. §21–5209 (2018 

Cum. Supp.). 

Under Kansas’ changed law, the defendant in Prosecution One could defend against the charge by arguing that

his mental illness prevented him from forming the mental

state required for murder (intentional killing of a human

being)—just as any defendant may attempt to rebut the

State’s prima facie case for guilt. The defendant in Prosecution Two has no defense. Because he acted with the requisite level of intent, he must be convicted regardless of any

role his mental illness played in his conduct. See 307 Kan. 

374, 401, 410 P. 3d 105, 125 (2018) (acknowledging that

Kansas’ mens rea approach “allows conviction of an individual who had no capacity to know that what he or she was 

doing was wrong”).

I do not mean to suggest that M’Naghten’s particular approach to insanity is constitutionally required. As we have 

said, “[h]istory shows no deference to M’Naghten.” Clark v. 

Arizona, 548 U. S. 735, 749 (2006). M’Naghten’s second 

prong is merely one way of describing something more fundamental. Its basic insight is that mental illness may so 

impair a person’s mental capacities as to render him no 

more responsible for his actions than a young child or a wild 

animal. Such a person is not properly the subject of the 

criminal law. As I shall explain in the following section, 
 

 

4 KAHLER v. KANSAS 

BREYER, J., dissenting 

throughout history, the law has attempted to embody this

principle in a variety of ways. As a historical matter, 

M’Naghten is by far its most prominent expression, but not

its exclusive one. Other ways of capturing it may well 

emerge in the future. The problem with Kansas’ law is that 

it excises this fundamental principle from its law entirely. 

II 

The Due Process Clause protects those “‘principle[s] of

justice so rooted in the traditions and conscience of our people as to be ranked as fundamental.’” Leland, 343 U. S., at 

798. Our “primary guide” in determining whether a principle of justice ranks as fundamental is “historical practice.” 

Montana v. Egelhoff, 518 U. S. 37, 43 (1996) (plurality opinion). The Court contends that the historical formulations 

of the insanity defense were so diverse, so contested, as to 

make it impossible to discern a unified principle that Kansas’ approach offends. I disagree.

Few doctrines are as deeply rooted in our common-law

heritage as the insanity defense. Although English and 

early American sources differ in their linguistic formulations of the legal test for insanity, with striking consistency,

they all express the same underlying idea: A defendant

who, due to mental illness, lacks sufficient mental capacity

to be held morally responsible for his actions cannot be

found guilty of a crime. This principle remained embedded 

in the law even as social mores shifted and medical understandings of mental illness evolved. Early American courts

incorporated it into their jurisprudence. The States eventually codified it in their criminal laws. And to this day, the

overwhelming majority of U. S. jurisdictions recognize insanity as an affirmative defense that excuses a defendant 

from criminal liability even where he was capable of forming the mens rea required for the offense. See Appendix, 

infra. 
 

 

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BREYER, J., dissenting 

A 

Consider the established common-law background of the 

insanity defense at and around the time the Framers wrote 

the Constitution. The four preeminent common-law jurists, 

Bracton, Coke, Hale, and Blackstone, each linked criminality to the presence of reason, free will, and moral understanding. It is “will and purpose,” wrote Henry de Bracton 

in his 13th-century treatise, that “mark maleficia [misdeeds].” 2 Bracton On Laws and Customs of England 384

(S. Thorne transl. 1968) (Bracton); Oxford Latin Dictionary 

1067 (P. Glare ed. 1982). A “madman,” he explained, “can

no more commit an injuria [unlawful conduct] or a felony 

than a brute animal, since they are not far removed from

brutes.” 2 Bracton 424; Oxford Latin Dictionary, at 914.

Seizing on Bracton’s reference to “brute animals” (sometimes translated “wild beasts”), the Court concludes that

Bracton’s approach, like Kansas’, would excuse only those 

who lack capacity to form any intention at all. See ante, at 

15. But what does it mean to be like a “brute animal”? A 

brute animal may well and readily intend to commit a violent act without being able to judge its moral nature. For 

example, when a lion stalks and kills its prey, though it acts

intentionally, it does not offend against the criminal laws. 

See 2 Bracton 379 (noting that “murder” is defined as “by 

the hand of man” to “distinguish it from the case of 

those slain or devoured by beasts and animals which lack 

reason”).

Bracton’s other references to “madmen” shed further 

light on the meaning he attached to that term. Bracton described such persons as “without sense and reason” and

“lack[ing] animus.” Id., at 324, 424. And he likened a “lunatic” to an “infant,” who cannot be held liable in damages 

unless he “is capable of perceiving the wrongful character 

of his act.” Id., at 324; see also 4 id., at 356 (“in many ways

a minor and a madman are considered equals or not very

different, because they lack reason” (footnote omitted)). 
 

6 KAHLER v. KANSAS 

BREYER, J., dissenting 

Thus, Bracton’s “brute animal” included those who lacked 

the qualities of reason and judgment that make human beings responsible moral agents. See Platt, The Origins and

Development of the “Wild Beast” Concept of Mental Illness

and Its Relation to Theories of Criminal Responsibility, 1 

Issues in Crim. 1, 6 (1965). 

Leaving Bracton, let us turn to Sir Edward Coke, writing

in the early 17th century. Coke wrote that “the act and 

wrong of a mad man shall not be imputed to him,” not because he could not engage in intentional conduct (the equivalent of the modern concept of mens rea), but because he

lacked something more—“mind or discretion.” 2 Institutes 

of the Laws of England §405, p. 247b (1628). Coke, like 

Bracton before him, likened a “mad man” to an “[i]nfant,” 

who could not be punished as a criminal “untill he be of the 

age of fourteene, which in Law is accounted the age of discretion.” Ibid. What is it that the “[i]nfant” lacks? Since 

long before Coke’s time, English jurists and scholars believed that it was the moral nature, not the physical nature,

of an act that a young child is unlikely to understand. See 

Platt & Diamond, The Origins of the “Right and Wrong” 

Test of Criminal Responsibility and Its Subsequent Development in the United States: An Historical Survey, 54 Cal.

L. Rev. 1227, 1233–1234 (1966) (Platt & Diamond). 

Sir Matthew Hale also premised criminal liability on the 

presence of “understanding and liberty of will,” without 

which “there can be no transgression, or just reason to incur 

the penalty or sanction that law instituted for the punishment of the crimes or offenses.” 1 Pleas of the Crown, ch. 2, 

pp. 14–15 (1736). Hale, too, likened insane persons to “infants” under the age of 14, who were subject to the criminal 

laws only if they “had discretion to judge between good and

evil.” Id., ch. 3, at 26–27; id., ch. 4, at 30 (a person who is

“labouring under melancholy distempers hath yet ordinarily as great understanding, as ordinarily a child of fourteen 

years hath, is such a person as may be guilty of treason or 
 

 

Cite as: 589 U. S. ____ (2020) 7 

BREYER, J., dissenting 

felony”). Those suffering from “total insanity” could not

be guilty of capital offenses, “for they have not the use of 

understanding, and act not as reasonable creatures, but 

their actions are in effect in the condition of brutes.” Id., 

at 30–32. 

Sir William Blackstone, whose influence on the founding 

generation was the most profound, was yet more explicit. A 

criminal offense, he explained, requires both a “vitious will” 

and a “vitious act.” 4 Commentaries on the Laws of England 21 (1769). Persons suffering from a “deficiency in will” 

arising from a “defective or vitiated understanding” were 

“not [criminally] chargeable for their own acts.” Id., at 24. 

Citing Coke, he explained that murder must be “committed

by a person of sound memory and discretion” because a “lunatic or infant” is “incapable of committing any crime, unless in such cases where they shew a consciousness of doing 

wrong, and of course a discretion, or discernment, between 

good and evil.” Id., at 195–196. And he opined that deprivation of “the capacity of discerning right from wrong” is 

necessary “to form a legal excuse.” Id., at 189. 

These four eminent jurists were not alone. Numerous 

other commentators expressly linked criminal liability with 

the accused’s capacity for moral agency. William Lambard’s 1581 treatise ranked a “mad man” as akin to a 

“childe” who had “no knowledge of good nor evil.” Eirenarcha, ch. 21, p. 218. If such a person killed a man, that is “no

felonious acte” because “they can[n]ot be said to have any 

understanding wil[l].” Ibid. But if “upon examination” it 

appeared that “they knew what they did, [and] it was ill, 

the[n] seemeth it to be otherwise.” Ibid. (emphasis added).

Michael Dalton’s 1618 manual for justices of the peace instructed that “[i]f one that is Non compos mentis . . . kill a 

man, this is no felonie; for they have no knowledge of good

and evill, nor can have a felonious intent, nor a will or mind 

to do harme.” The Countrey Justice 215. William Hawkins, 
 

8 KAHLER v. KANSAS 

BREYER, J., dissenting 

in 1716, wrote that “those who,” like “[l]unaticks,” are “under a natural Disability of distinguishing between Good and 

Evil . . . are not punishable by any criminal Prosecution

whatsoever.” 1 Pleas of the Crown §1, p. 2; see also id., at 

1 (“The Guilt of offending against any Law whatsoever . . . 

can never justly be imputed to those who are either uncapable of understanding it, or of conforming themselves to

it”).

English treatises on the law of mental disability adopted 

the same view. George Collinson explained that “[t]o excuse a man in the commission of a crime, he must at the 

period when he committed the offense, have been wholly incapable of distinguishing between good and evil, or of comprehending the nature of what he is doing.” Treatise on the 

Law Concerning Idiots, Lunatics, and Other Persons Non

Compotes Mentis §7, p. 474 (1812) (Collinson); see also id., 

§2, at 471 (“[A]n evil intention is implied in every offence, 

and constitutes the charge of every indictment: but a non

compos, not having a will of his own, cannot have an intention morally good or bad; so that the overt act by which

alone the motives of other men are discerned, with respect 

to him proves nothing”). Similarly, Leonard Shelford, summarizing English case law, wrote that “[t]he essence of a 

crime consists in the animus or intention of the person who

commits it, considered as a free agent, and in a capacity of 

distinguishing between moral good and evil.” Practical 

Treatise on the Law Concerning Lunatics, Idiots, and Persons of Unsound Mind 458 (1833) (emphasis deleted).

The majority believes that I am “cherry-pick[ing]” references to moral understanding while ignoring references to 

intent and mens rea. See ante, at 15–17, nn. 8, 9. With 

respect, I disagree. The Court points out, correctly, that 

many of the common-law sources state that the insane lack 

mens rea or felonious intent. But what did they mean by 

that? At common law, the term mens rea ordinarily incor-
 

 

Cite as: 589 U. S. ____ (2020) 9 

BREYER, J., dissenting 

porated the notion of “general moral blameworthiness” required for criminal punishment. Sayre, Mens Rea, 45 Harv. 

L. Rev. 974, 988 (1932); 3 Encyclopedia of Crime and Justice 995 (2d ed. 2002) (as used at common law, the term 

mens rea “is synonymous with a person’s blameworthiness”). The modern meaning of mens rea is narrower and 

more technical. Ibid. It refers to the “state of mind or inattention that, together with its accompanying conduct, the 

criminal law defines as an offense.” Ibid. When commonlaw writers speak of intent or mens rea, we cannot simply 

assume that they use those terms in the modern sense. 

That is an anachronism. Instead, we must examine the 

context to understand what meaning they ascribed to those 

terms. And when we do so, we see that, over and over again, 

they link criminal intent to the presence of free will and

moral understanding. The Court dismisses those passages

as just “some ‘good and evil’ language.” Ante, at 17, n. 9. 

But it fails to explain why, if mens rea in the modern sense 

were sufficient, these common-law writers discuss the role 

of moral agency at all, much less why such language appears in virtually every treatise and virtually every case.

In the Court’s view, all that is just spilled ink.

The English case law illustrates this point. In the seminal case of Rex v. Arnold, 16 How. St. Tr. 695 (1724), the 

defendant stood accused of shooting Lord Onslow while laboring under the insane delusion that Onslow had bewitched him. Id., at 699, 721. The Court emphasizes Justice Tracy’s statement to the jury that if a man is “‘deprived

of his reason, and consequently of his intention, he cannot 

be guilty,’” concluding that the court adopted a modern 

mens rea test. Ante, at 16. But in the passage immediately 

preceding that statement, Justice Tracy explained that the 

defendant’s intent to shoot was clearly proved, and that the 

only remaining question was whether his mental illness excused him from blame: 
 

 

10 KAHLER v. KANSAS 

BREYER, J., dissenting 

“That he shot, and that wilfully [is proved]: but

whether maliciously, that is the thing: that is the question; whether this man hath the use of his reason and 

sense? If he was under the visitation of God, and could 

not distinguish between good and evil, and did not

know what he did, though he committed the greatest

offence, yet he could not be guilty of any offence against 

any law whatsoever; for guilt arises from the mind, and 

the wicked will and intention of the man. If a man be 

deprived of his reason, and consequently of his intention, he cannot be guilty; and if that be the case, though

he had actually killed my lord Onslow, he is exempted 

from punishment.” 16 How. St. Tr., at 764 (emphasis

added; brackets in original). 

See also ibid. (summarizing the testimony of one Mr. Coe, 

who testified that he went to the defendant three days after

the shooting “and asked him, If he intended to kill my lord

Onslow? and he said, Yes, to be sure”). On the next page,

Justice Tracy concluded that the jury must determine

whether the evidence “doth shew a man, who knew what he 

was doing, and was able to distinguish whether he was doing good or evil, and understood what he did.” Id., at 765. 

Likewise, in the case of Rex v. Lord Ferrers, 19 How. St. 

Tr. 886 (1760), the solicitor general instructed the members

of the House of Lords to consider the “‘capacity and intention’” of the accused, to be sure, ante, at 17, but what did he 

mean by those terms? The ultimate question of insanity,

he explained, depended on the defendant’s capacity at the

time of the offense to distinguish right from wrong: 

“My lords, the question therefore must be asked; is the 

noble prisoner at the bar to be acquitted from the guilt

of murder, on account of insanity? It is not pretended

to be a constant general insanity. Was he under the 

power of it, at the time of the offence committed? Could 

he, did he, at that time, distinguish between good and 
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BREYER, J., dissenting 

evil?” 19 How. St. Tr., at 948. 

In summation, the solicitor general argued that Lord Ferrers’ own witnesses failed to provide any testimony “which 

proves his lunacy or insanity at any time.” Id., at 952. Reviewing the pertinent evidence, he noted that one witness 

testified that he “had observed great oddities in my lord,” 

but acknowledged that he “never saw him in such a situation, as not to be capable of distinguishing between good 

and evil, and not to know, that murder was a great crime.” 

Ibid. Another admitted under questioning by the Lords

that “he thought lord Ferrers capable of distinguishing between moral and immoral actions.” Ibid. The defendant’s 

brother was the only witness to testify that “at particular

times, the noble lord might not be able to distinguish between moral good and evil,” but even he, the solicitor general argued, had been unable to testify to “any instance

within his own recollection.” Id., at 953. If Lord Ferrers’ 

bare intention to kill were sufficient to convict, why the extensive discussion of the evidence concerning his capacity 

for moral understanding?

These examples reflect the prevailing view of the law 

around the time of the founding. Judges regularly instructed juries that the defendant’s criminal liability depended on his capacity for moral responsibility. See, e.g., 

Trial of Samuel Burt (July 19, 1786), in 6 Old Bailey Proceedings 875 (E. Hodgson ed. 1788) (to acquit based on insanity, it must be shown that the mental disorder “takes

away from the party all moral agency and accountability,” 

and “destroys in them, for the time at least, all power of 

judging between right and wrong”); Trial of Francis Parr 

(Jan. 15, 1787), 2 id., at 228 (jury must “judge whether at

the moment of committing [the offense] he was not a moral

agent, capable of discerning between good and evil, and of 

knowing the consequences of what he did”); Bowler’s Case, 

1 Collinson 673–674, n. (judge “concluded by observing to 
 

12 KAHLER v. KANSAS 

BREYER, J., dissenting 

the jury, that it was for them to determine whether the 

Prisoner, when he committed the offence with which he 

stood charged, was or was not incapable of distinguishing 

right from wrong”). The government’s attorneys agreed

that this was the proper inquiry. See, e.g., Parker’s Case, 1 

id., at 479–480 (the Attorney General argued that “the jury

must be perfectly satisfied, that at the time when the crime

was committed, the prisoner did not really know right from

wrong”).

In none of the common-law cases was the judge’s reference to the defendant’s capacity for moral agency simply a

proxy for the narrow modern notion of mens rea. See ante, 

at 17. Something more was required. Consider Bellingham’s Case, 1 Collinson 636. The defendant stood accused 

of the murder of Spencer Perceval, the Chancellor of the Exchequer, in the lobby of the House of Commons. Ibid. The 

Court emphasizes Chief Justice Mansfield’s statement that 

one who could not distinguish right from wrong “‘could have 

no intention at all,’” concluding that Chief Justice Mansfield viewed moral incapacity as a symptom of cognitive

breakdown rather than a test of insanity. Ante, at 18. But, 

as in Rex v. Arnold, see supra, at 9–10, the defendant’s intention to shoot Perceval was not seriously in dispute. 1 

Collinson 670. Instead, his guilt or innocence turned on his 

capacity for moral blame. The “single question” for the jury,

charged the Chief Justice, “was whether, when [the defendant] committed the offence charged upon him, he had sufficient understanding to distinguish good from evil, right 

from wrong, and that murder was a crime not only against 

the law of God, but against the law of his Country.” Id., at 

673. Lord Lyndhurst, presiding over the case of Rex v. Offord, 5 Car. & P. 168, 172 Eng. Rep. 924 (N. P. 1831), certainly understood that inquiry to be the crux of Chief Justice Mansfield’s charge. Citing Bellingham’s Case, he 

instructed the jury that “[t]he question was, did [the accused] know that he was committing an offence against the 
Cite as: 589 U. S. ____ (2020) 13 

BREYER, J., dissenting 

laws of God and nature?” 5 Car. & P., at 168, 172 Eng. Rep., 

at 925. 

The Court dismisses other common-law cases as failing

to articulate a clear legal standard. See ante, at 18–19. But 

these cases, too, required more than bare intent. In Hadfield’s Case, 27 How. St. Tr. 1281 (1800), the defendant was

acquitted after the prosecution conceded that he was “in a 

deranged state of mind” when he shot at King George III. 

Id., at 1353. And in Regina v. Oxford, 9 Car. & P. 525, 173 

Eng. Rep. 941 (N. P. 1840), the court observed that a “person may commit a criminal act, and yet not be responsible.” 

Id., at 546, 173 Eng. Rep., at 950. Although it acknowledged the difficulty of “lay[ing] down the rule of the English 

law on the subject,” it summed up the inquiry as “whether 

the prisoner was labouring under that species of insanity 

which satisfies you that he was quite unaware of the nature, character, and consequences of the act he was committing, or, in other words, whether he was under the influence

of a diseased mind, and was really unconscious at the time

he was committing the act, that it was a crime.” Id., at 546– 

547, 173 Eng. Rep., at 950. Although these and other English cases discuss insanity in terms that are less precise

than our modern taxonomy of mental states, their lesson is

clear. To be guilty of a crime, the accused must have something more than bare ability to form intentions and carry 

them out. 

B 

These fundamental principles of criminal responsibility

were incorporated into American law from the early days of 

the Republic. Early American commentaries on the criminal law generally consisted of abridgments of the works of

prominent English jurists. As early as 1792, one such

abridgment instructed that “lunaticks, who are under a 

natural disability of distinguishing between good and evil 

are not punishable by any criminal prosecution.” R. Burn, 
 

 

 

14 KAHLER v. KANSAS 

BREYER, J., dissenting 

Abridgment, or the American Justice 300; see also W. 

Stubbs, Crown Circuit Companion 288 (1 Am. ed. 1816) (“If 

one that is non compos mentis . . . kill a man, this is no felony; for they have not knowledge of good and evil, nor can 

have a felonious intent, nor a will or mind to do harm”).

And an influential founding-era legal dictionary described 

the “general rule” that lunatics, “being by reason of their 

natural disabilities incapable of judging between good and 

evil, are punishable by no criminal prosecution whatsoever.” 2 T. Cunningham, New and Complete Law

Dictionary (2d corr. ed. 1771). Similarly, the first comprehensive American text on forensic medicine, published in 

1823, cited Chief Justice Mansfield’s charge to the jury in 

Bellingham’s Case for the proposition that “[s]o long as they 

could distinguish good from evil, so long would they be answerable for their conduct.” 1 T. Beck, Elements of Medical 

Jurisprudence 369. These principles, it concluded, “are

doubtless correct, and conducive to the ends of justice.” Id., 

at 370. 

Early American jurists closely hewed to these principles.

In case after case, judges instructed juries that they must

inquire into the defendant’s capacity for moral understanding. See, e.g., Meriam’s Case, 7 Mass. 168 (1810), 6 N. Y. 

City-Hall Recorder 162 (1822) (whether the defendant was 

“at the time, capable of distinguishing good from evil”); 

Clark’s Case, 1 N. Y. City-Hall Recorder 176, 177 (1816) 

(same); Ball’s Case, 2 N. Y. City-Hall Recorder 85, 86 (1817) 

(same); United States v. Clarke, 25 F. Cas. 454 (No. 14,811)

(CC DC 1818) (whether defendant was “in such a state of 

mental insanity . . . as not to have been conscious of the 

moral turpitude of the act”); Cornwell v. State, 8 Tenn. 147, 

155 (1827) (whether the prisoner “had not sufficient understanding to know right from wrong”). 

C 

As the foregoing demonstrates, by the time the House of 
Cite as: 589 U. S. ____ (2020) 15 

BREYER, J., dissenting 

Lords articulated the M’Naghten test in 1843, its “essential 

concept and phraseology” were “already ancient and thoroughly embedded in the law.” Platt & Diamond 1258; see 

also 1 W. Russell, Crimes and Misdemeanors 8–14 (3d ed.

1843) (summarizing the pre-M’Naghten English case law

and concluding that the key questions were whether “there

be thought and design, a faculty to distinguish the nature

of actions, [and] to discern the difference between moral 

good and evil”). Variations on the M’Naghten rules soon 

became the predominant standard in the existing states of

the United States. Platt & Diamond 1257. That tradition 

has continued, almost without exception, to the present 

day.

It is true that, even following M’Naghten, States continued to experiment with different formulations of the insanity defense. See ante, at 19–20. Some adopted the volitional 

incapacity, or “irresistible-impulse,” test. But those States 

understood that innovation to expand, not contract, the 

scope of the insanity defense, excusing not only defendants

who met some variant of the traditional M’Naghten test but 

also those who understood that their conduct was wrong but

were incapable of restraint. See, e.g., Parsons v. State, 81 

Ala. 577, 584–585, 2 So. 854, 858–859 (1887); Bradley v. 

State, 31 Ind. 492, 507–508 (1869); State v. Felter, 25 Iowa 

67, 82–83 (1868); Hopps v. People, 31 Ill. 385, 391–392 

(1863).

So too, the “offspring” or “product” test, which asks 

whether the defendant’s conduct was attributable to mental 

disease or defect. The States that adopted this test did so

out of the conviction that the M’Naghten test was too restrictive in its approach to assessing the accused’s capacity

for criminal responsibility. See Durham v. United States, 

214 F. 2d 862, 874 (CADC 1954) (“We conclude that a

broader test should be adopted”); State v. Pike, 49 N. H. 399, 

441–442 (1870); see also Reid, Understanding the New 

Hampshire Doctrine of Criminal Insanity, 69 Yale L. J. 367, 
 

 

 

16 KAHLER v. KANSAS 

BREYER, J., dissenting 

386 (1960) (“[T]he New Hampshire doctrine . . . is more liberal and has a wider range than M’Naghten rules”). Even 

as States experimented with broader insanity rules, they

retained the core of the traditional common-law defense. 

In the early 20th century, several States attempted to

break with that tradition. The high courts of those States

quickly struck down their restrictive laws. As one justice of 

the Mississippi Supreme Court wrote in 1931: The “common 

law proceeds upon an idea that before there can be a crime

there must be an intelligence capable of comprehending the

act prohibited, and the probable consequence of the act, and

that the act is wrong.” Sinclair v. State, 161 Miss. 142, 158, 

132 So. 581, 583 (Ethridge, J., concurring). Accordingly,

Justice Ethridge said, insanity “has always been a complete

defense to all crimes from the earliest ages of the common 

law.” Ibid.; State v. Strasburg, 60 Wash. 106, 116, 110 P. 

1020, 1022–1023 (1910); cf. State v. Lange, 168 La. 958, 965, 

123 So. 639, 642 (1929).

Today, 45 States, the Federal Government, and the District of Columbia continue to recognize an insanity defense 

that retains some inquiry into the blameworthiness of the 

accused. Seventeen States and the Federal Government 

use variants of the M’Naghten test, with its alternative cognitive and moral incapacity prongs. Three States have 

adopted M’Naghten plus the volitional test. Ten States recognize a defense based on moral incapacity alone. Thirteen 

States and the District of Columbia have adopted variants 

of the Model Penal Code test, which combines volitional incapacity with an expanded version of moral incapacity. See 

Appendix, infra. New Hampshire alone continues to use 

the “product” test, asking whether “a mental disease or defect caused the charged conduct.” State v. Fichera, 153 

N. H. 588, 593, 903 A. 2d 1030, 1035 (2006). This broad test 

encompasses “‘whether the defendant knew the difference 

between right and wrong and whether the defendant acted 
Cite as: 589 U. S. ____ (2020) 17 

BREYER, J., dissenting 

impulsively,’” as well as “‘whether the defendant was suffering from delusions or hallucinations.’” State v. Cegelis, 

138 N. H. 249, 255, 638 A. 2d 783, 786 (1994). And North 

Dakota uses a unique formulation that asks whether the

defendant “lacks substantial capacity to comprehend the

harmful nature or consequences of the conduct, or the conduct is the result of a loss or serious distortion of the individual’s capacity to recognize reality.” N. D. Cent. Code 

Ann. §12.1–04.1–01(1) (2012).

Of the States that have adopted the M’Naghten or Model 

Penal Code tests, some interpret knowledge of wrongfulness to refer to moral wrong, whereas others hold that it

means legal wrong. See ante, at 2–3, 20–22. While there 

is, of course, a logical distinction between those interpretations, there is no indication that it makes a meaningful difference in practice. The two inquiries are closely related 

and excuse roughly the same universe of defendants. See 

State v. Worlock, 117 N. J. 596, 609–611, 569 A. 2d 1314, 

1321–1322 (1990) (“In most instances, legal wrong is coextensive with moral wrong”); State v. Crenshaw, 98 Wash. 2d 

789, 799, 659 P. 2d 488, 494 (1983) (“ ‘[S]ince by far the vast 

majority of cases in which insanity is pleaded as a defense 

to criminal prosecutions involves acts which are universally 

recognized as morally wicked as well as illegal, the hairsplitting distinction between legal and moral wrong need

not be given much attention’ ”); People v. Schmidt, 216 N. Y. 

324, 340, 110 N. E. 945, 949 (1915) (Cardozo, J.) 

(“Knowledge that an act is forbidden by law will in most

cases permit the inference of knowledge that, according to

the accepted standards of mankind, it is also condemned as 

an offense against good morals”); see also ALI, Model Penal 

Code §4.01, Explanatory Note, p. 164 (1985) (explaining 

that “few cases are likely to arise in which the variation will

be determinative”). 
 

 

18 KAHLER v. KANSAS 

BREYER, J., dissenting 

III 

A 

Consider the basic reason that underlies and explains 

this long legal tradition. That reason reveals that more is 

at stake than its duration alone. The tradition reflects the 

fact that a community’s moral code informs its criminal law.

As Henry Hart stated it, the very definition of crime is conduct that merits “a formal and solemn pronouncement of

the moral condemnation of the community.” The Aims of 

the Criminal Law, 23 Law & Contemp. Prob. 401, 405

(1958).

The criminal law does not adopt, nor does it perfectly

track, moral law. It is no defense simply to claim that one’s 

criminal conduct was morally right. But the criminal law 

nonetheless tries in various ways to prevent the distance

between criminal law and morality from becoming too 

great. In the words of Justice Holmes, a law that “punished 

conduct [that] would not be blameworthy in the average

member of the community would be too severe for that community to bear.” O. Holmes, The Common Law 50 (1881); 

see also ibid. (“[T]o deny that criminal liability . . . is 

founded on blameworthiness . . . would shock the moral 

sense of any civilized community”). 

Sometimes the criminal law seeks to keep its strictures

roughly in line with the demands of morality through

grants of discretion that will help it to reach appropriate

results in individual cases, including special instances

where the law points one way and morality the other. Thus, 

prosecutors need not prosecute. Jurors (however instructed) may decide to acquit. Judges may exercise the

discretion the law allows them to impose a lenient sentence. 

Executives may grant clemency. 

And sometimes the law attempts to maintain this balance by developing and retaining a “collection of interlocking and overlapping concepts,” including defenses, that will 

help “assess the moral accountability of an individual for 
 

Cite as: 589 U. S. ____ (2020) 19 

BREYER, J., dissenting 

his antisocial deeds.” Powell v. Texas, 392 U. S. 514, 535– 

536 (1968) (plurality opinion). These concepts and defenses 

include “actus reus, mens rea, insanity, mistake, justification, and duress.” Id., at 536. 

As we have recognized, the “process of adjustment”

within and among these overlapping legal concepts “has always been thought to be the province of the States.” Ibid. 

Matters of degree, specific content, and aptness of application all may be, and have always been, the subject of legal

dispute. But the general purpose—to ensure a rough congruence between the criminal law and widely accepted

moral sentiments—persists. To gravely undermine the insanity defense is to pose a significant obstacle to this basic 

objective.

The majority responds that Kansas has not removed the

element of blameworthiness from its treatment of insanity; 

it has simply made a different judgment about what conduct is blameworthy. See ante, at 13, n. 7. That is not how 

the Kansas Supreme Court has characterized its law. See 

State v. Bethel, 275 Kan. 456, 472, 66 P. 3d 840, 850 (2003) 

(holding that Kansas law provides for “no consideration,” at 

the guilt phase, “of whether wrongfulness was inherent in

the defendant’s intent”). In any event, as the Court

acknowledges, the States’ discretion in this area must be 

constrained within “broad limits,” ante, at 7, which are derived from history and tradition. The question is whether

Kansas’ approach transgresses those limits. I doubt that 

the Court would declare, for example, that a State may do

away with the defenses of duress or self-defense on the 

ground that, in its idiosyncratic judgment, they are not required. With respect to the defense of insanity, I believe

that our history shows clearly that the criminal law has always required a higher degree of individual culpability than 

the modern concept of mens rea. See Part II, supra. And in 

my view, Kansas’ departure from this long uniform tradition poses a serious problem. 
 

 

 

20 KAHLER v. KANSAS 

BREYER, J., dissenting 

B 

To see why Kansas’ departure is so serious, go back to our 

two simplified prosecutions: the first of the defendant who,

because of serious mental illness, believes the victim is a 

dog; the second of a defendant who, because of serious mental illness, believes the dog commanded him to kill the victim. Now ask, what moral difference exists between the defendants in the two examples? Assuming equivalently

convincing evidence of mental illness, I can find none at all.

In both cases, the defendants differ from ordinary persons 

in ways that would lead most of us to say that they should

not be held morally responsible for their acts. I cannot find 

one defendant more responsible than the other. And for 

centuries, neither has the law. 

More than that, scholars who have studied this subject

tell us that examples of the first kind are rare. See Brief 

for 290 Criminal Law and Mental Health Law Professors as 

Amici Curiae 12. Others repeat this claim. See Slobogin,

An End to Insanity: Recasting the Role of Mental Disability 

in Criminal Cases, 86 Va. L. Rev. 1199, 1205 (2000); Morse, 

Mental Disorder and Criminal Law, 101 J. Crim. L. & C. 

885, 933 (2011). That is because mental illness typically

does not deprive individuals of the ability to form intent. 

Rather, it affects their motivations for forming such intent.

Brief for 290 Criminal Law and Mental Health Law Professors as Amici Curiae 12. For example, the American Psychiatric Association tells us that individuals suffering from 

mental illness may experience delusions—erroneous perceptions of the outside world held with strong conviction. 

They may believe, incorrectly, that others are threatening

them harm (persecutory delusions), that God has commanded them to engage in certain conduct (religious delusions), or that they or others are condemned to a life of suffering (depressive delusions). Brief for American 

Psychiatric Association et al. as Amici Curiae 25–26. Such 

delusions may, in some cases, lead the patient to behave 
 

Cite as: 589 U. S. ____ (2020) 21 

BREYER, J., dissenting 

violently. Id., at 28. But they likely would not interfere 

with his or her perception in such a way as to negate mens 

rea. See H. R. Rep. No. 98–577, p. 15 n. 23 (1984) (“Mental

illness rarely, if ever, renders a person incapable of understanding what he or she is doing. Mental illness does not, 

for example, alter the perception of shooting a person to 

that of shooting a tree.”).

Kansas’ abolition of the second part of the M’Naghten test 

requires conviction of a broad swath of defendants who are 

obviously insane and would be adjudged not guilty under

any traditional form of the defense. This result offends 

deeply entrenched and widely recognized moral principles

underpinning our criminal laws. See, e.g., National 

Comm’n on Reform of Fed. Crim. Laws, Final Report, Proposed New Fed. Crim. Code §503, pp. 40–41 (1971) (to attribute guilt to a “manifestly psychotic person” would “be 

immoral and inconsistent with the aim of a criminal code”); 

H. R. Rep. No. 98–577, at 7–8 (“[T]he abolition of the affirmative insanity defense would alter that fundamental basis 

of Anglo-American criminal law: the existence of moral culpability as a prerequisite for punishment”); ABA Criminal

Justice Mental Health Standards §7–6.1, pp. 336–338 

(1989) (rejecting the mens rea approach “out of hand” as “a

jarring reversal of hundreds of years of moral and legal

history” that “inhibits if not prevents the exercise of humane judgment that has distinguished our criminal law 

heritage”).

By contrast, the rule adopted by some States that a defendant must be acquitted if he was unable to appreciate 

the legal wrongfulness of his acts, see ante, at 20–22, would 

likely lead to acquittal in the mine run of such cases. See 

supra, at 17. If that is so, then that rule would not pose the

same due process problem as Kansas’ approach. That issue 

is not before us, as Kansas’ law does not provide even that 

protection to mentally ill defendants. 
 

 

22 KAHLER v. KANSAS 

BREYER, J., dissenting 

C 

Kansas and the Solicitor General, in their efforts to justify Kansas’ change, make four important arguments.

First, they point to cases in this Court in which we have 

said that the States have broad leeway in shaping the insanity defense. See Leland, 343 U. S. 790; Clark, 548 U. S. 

735. In Leland, we rejected the defendant’s argument that

the Constitution required the adoption of the “‘irresistible 

impulse’” test. 343 U. S., at 800–801. Similarly, in Clark, 

we upheld Arizona’s effort to eliminate the first part of the 

M’Naghten rule, applicable to defendants whose mental illness deprived them of the ability to know the “‘nature and 

quality of the act,’” 548 U. S., at 747–748. If Arizona can 

eliminate the first prong of M’Naghten, Kansas asks, why 

can Kansas not eliminate the second part? 

The answer to this question lies in the fact that Arizona,

while amending the insanity provisions of its criminal code,

did not in practice eliminate the traditional insanity defense in any significant part. See 548 U. S., at 752, n. 20 

(reserving the question whether “the Constitution mandates an insanity defense”). As we pointed out, “cognitive 

incapacity is itself enough to demonstrate moral incapacity.” Id., at 753. Evidence that the defendant did not know 

what he was doing would also tend to establish that he did 

not know that it was wrong. Id., at 753–754. And Prosecution One (he thought the victim was a dog) would still fail. 

The ability of the States to refuse to adopt other insanity

tests, such as the “irresistible impulse” test or the “product

of mental illness” test are also beside the point. See Leland, 

343 U. S., at 800–801. Those tests both expand upon 

M’Naghten’s principles. Their elimination would cut the 

defense back to what it traditionally has been, not, as here, 

eliminate its very essence. 

Second, the United States as amicus curiae suggests that

the insanity defense is simply too difficult for juries to administer. Brief for United States as Amicus Curiae 12–13. 
 

Cite as: 589 U. S. ____ (2020) 23 

BREYER, J., dissenting 

Without doubt, assessing the defendant’s claim of insanity 

is difficult. That is one reason I believe that States must 

remain free to refine and redefine their insanity rules 

within broad bounds. But juries have been making that determination for centuries and continue to do so in 45 States. 

And I do not see how an administrative difficulty can justify 

abolishing the heart of the defense.

Third, Kansas argues that it has not abolished the insanity defense or any significant part of it. It has simply moved 

the stage at which a defendant can present the full range of 

mental-capacity evidence to sentencing. See Brief for Respondent 8; ante, at 4–5. But our tradition demands that 

an insane defendant should not be found guilty in the first 

place. Moreover, the relief that Kansas offers, in the form 

of sentencing discretion and the possibility of commitment

in lieu of incarceration, is a matter of judicial discretion, not 

of right. See State v. Maestas, 298 Kan. 765, 316 P. 3d 724 

(2014). The insane defendant is, under Kansas law, exposed to harsh criminal sanctions up to and including

death. And Kansas’ sentencing provisions do nothing to alleviate the stigma and the collateral consequences of a 

criminal conviction. 

Finally, Kansas argues that the insane, provided they are

capable of intentional action, are culpable and should be 

held liable for their antisocial conduct. Brief for Respondent 40. To say this, however, is simply to restate the conclusion for which Kansas argues in this case. It is a conclusion that in my view runs contrary to a legal tradition that

embodies a fundamental precept of our criminal law and 

that stretches back, at least, to the origins of our Nation. 

For these reasons, with respect, I dissent. 
 

 

 

BREYER, J., dissenting 

24 KAHLER v. KANSAS 

Appendix to the opinion of BREYER, J. 

APPENDIX 

M’Naghten 

State Text 

Alabama “It is an affirmative defense to a prosecution for any 

crime that, at the time of the commission of the acts constituting the offense, the defendant, as a result of severe

mental disease or defect, was unable to appreciate the nature and quality or wrongfulness of his acts.” Ala. Code 

§13A–3–1(a) (2015). 

California “In any criminal proceeding, including any juvenile 

court proceeding, in which a plea of not guilty by reason 

of insanity is entered, this defense shall be found by the

trier of fact only when the accused person proves by a preponderance of the evidence that he or she was incapable

of knowing or understanding the nature and quality of his 

or her act and of distinguishing right from wrong at the 

time of the commission of the offense.” Cal. Penal Code 

Ann. §25(b) (West 2014). 

Colorado “(1) The applicable test of insanity shall be:

“(a) A person who is so diseased or defective in mind at 

the time of the commission of the act as to be incapable of

distinguishing right from wrong with respect to that act 

is not accountable; except that care should be taken not 

to confuse such mental disease or defect with moral obliquity, mental depravity, or passion growing out of anger,

revenge, hatred, or other motives and kindred evil conditions, for, when the act is induced by any of these causes,

the person is accountable to the law; or

“(b) A person who suffered from a condition of mind 

caused by mental disease or defect that prevented the 

person from forming a culpable mental state that is an 

essential element of a crime charged, but care should be 

taken not to confuse such mental disease or defect with 

moral obliquity, mental depravity, or passion growing out

of anger, revenge, hatred, or other motives and kindred 

evil conditions because, when the act is induced by any of 

these causes, the person is accountable to the law.” Colo. 

Rev. Stat. §16–8–101.5(1) (2019). 

Florida “(1) AFFIRMATIVE DEFENSE.––All persons are presumed to be sane. It is an affirmative defense to a criminal prosecution that, at the time of the commission of the 

acts constituting the offense, the defendant was insane. 

Insanity is established when: 

“(a) The defendant had a mental infirmity, disease, or

defect; and

“(b) Because of this condition, the defendant: 
BREYER, J., dissenting 

Cite as: 589 U. S. ____ (2020) 25 

Appendix to the opinion of BREYER, J. 

State Text 

“1. Did not know what he or she was doing or its consequences; or

“2. Although the defendant knew what he or she was

doing and its consequences, the defendant did not know 

that what he or she was doing was wrong. 

“Mental infirmity, disease, or defect does not constitute a 

defense of insanity except as provided in this subsection.” 

Fla. Stat. §775.027 (2018). 

Iowa “A person shall not be convicted of a crime if at the time 

the crime is committed the person suffers from such a diseased or deranged condition of the mind as to render the

person incapable of knowing the nature and quality of the 

act the person is committing or incapable of distinguishing between right and wrong in relation to that act.” Iowa 

Code §701.4 (2016). 

Minne- “No person having a mental illness or cognitive impairsota ment so as to be incapable of understanding the proceedings or making a defense shall be tried, sentenced, or punished for any crime; but the person shall not be excused 

from criminal liability except upon proof that at the time

of committing the alleged criminal act the person was laboring under such a defect of reason, from one of these 

causes, as not to know the nature of the act, or that it was 

wrong.” Minn. Stat. §611.026 (2019). 

Missis- “In determining sanity in criminal cases Mississippi utisippi lizes the common law M’Naghten test. Under the 

M’Naghten test, the accused must be laboring under such 

defect of reason from disease of the mind as (1) not to

know the nature and quality of the act he was doing or (2) 

if he did not know it, that he did not know that what he 

was doing was wrong.” Parker v. State, 273 So. 3d 695, 

705–706 (Miss. 2019) (internal quotation marks and footnote omitted).

Missouri “A person is not responsible for criminal conduct if at the

time of such conduct as a result of mental disease or defect he was incapable of knowing and appreciating the nature, quality or wrongfulness of his or her conduct.” Mo. 

Rev. Stat. §562.086(1) (2016). 

Nebraska “Under our current common-law definition, the two requirements for the insanity defense are that (1) the defendant had a mental disease or defect at the time of the 

crime and (2) the defendant did not know or understand

the nature and consequences of his or her actions or that 

he or she did not know the difference between right and

wrong.” State v. Hotz, 281 Neb. 260, 270, 795 N. W. 2d 
 

 

 

BREYER, J., dissenting 

26 KAHLER v. KANSAS 

Appendix to the opinion of BREYER, J. 

State Text 

645, 653 (2011). 

Nevada “To qualify as being legally insane, a defendant must be 

in a delusional state such that he cannot know or understand the nature and capacity of his act, or his delusion 

must be such that he cannot appreciate the wrongfulness

of his act, that is, that the act is not authorized by law.” 

Finger v. State, 117 Nev. 548, 576, 27 P. 3d 66, 84–85 

(2001). 

New “A person is not criminally responsible for conduct if at

Jersey the time of such conduct he 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.” N. J. Stat. Ann. §2C:4–1 (West 2015). 

New York “In any prosecution for an offense, it is an affirmative

defense that when the defendant engaged in the proscribed conduct, he lacked criminal responsibility by reason of mental disease or defect. Such lack of criminal responsibility means that at the time of such conduct, as a 

result of mental disease or defect, he lacked substantial 

capacity to know or appreciate either: 

“1. The nature and consequences of such conduct; or

“2. That such conduct was wrong.” N. Y. Penal Law 

Ann. §40.15 (West 2009). 

North “[A]n accused is legally insane and exempt from criminal 

Carolina responsibility by reason thereof if he commits an act 

which would otherwise be punishable as a crime, and at 

the time of so doing is laboring under such a defect of reason, from disease of the mind, as to be incapable of knowing the nature and quality of the act he is doing, or, if he

does know this, incapable of distinguishing between right

and wrong in relation to such act.” State v. Thompson,

328 N. C. 477, 485–486, 402 S. E. 2d 386, 390 (1991). 

Oklahoma “Oklahoma uses the M’Naghten test to determine the issue of sanity at the time of the crime. This Court has held 

that the M’Naghten insanity test, as applied in Oklahoma, has two prongs. Under the first prong, the defendant is considered insane if he is suffering from a mental 

disability such that he does not know his acts are wrong

and he is unable to distinguish right from wrong with respect to his acts. Under the second prong, the defendant 

is considered insane if suffering from a disability of reason or disease of the mind such that he does not understand the nature or consequences of his acts or omissions.

The defendant need only satisfy one of these prongs in 

order to be found not guilty by reason of insanity.” 
 

 

 

 

 

BREYER, J., dissenting 

Cite as: 589 U. S. ____ (2020) 27 

Appendix to the opinion of BREYER, J. 

State Text 

Cheney v. State, 909 P. 2d 74, 90 (Okla. 1995) (footnotes 

omitted).

Pennsyl- “Common law M’Naghten’s Rule preserved.—Nothing in

vania this section shall be deemed to repeal or otherwise abrogate the common law defense of insanity (M’Naghten’s 

Rule) in effect in this Commonwealth on the effective date

of this section.” 18 Pa. Cons. Stat. §314(d) (2015). 

Tennes- “It is an affirmative defense to prosecution that, at the 

see 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 or 

wrongfulness of the defendant’s acts.” Tenn. Code Ann. 

§39–11–501(a) (2018). 

Washing- “To establish the defense of insanity, it must be shown 

ton that: 

“(1) At the time of the commission of the offense, as a 

result of mental disease or defect, the mind of the actor

was affected to such an extent that: 

“(a) He or she was unable to perceive the nature and

quality of the act with which he or she is charged; or

“(b) He or she was unable to tell right from wrong with 

reference to the particular act charged.” Wash. Rev. Code 

§9A.12.010 (2015). 

Federal “Affirmative Defense.—It is an affirmative defense to 

a prosecution under any Federal statute 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. 

M’Naghten plus volitional incapacity

State Text 

Georgia “A person shall not be found guilty of a crime if, at the 

time of the act, omission, or negligence constituting the 

crime, the person did not have mental capacity to distinguish between right and wrong in relation to such act, 

omission, or negligence.” Ga. Code Ann. §16–3–2 (2019).

“A person shall not be found guilty of a crime when, at 

the time of the act, omission, or negligence constituting 

the crime, the person, because of mental disease, injury,

or congenital deficiency, acted as he did because of a delusional compulsion as to such act which overmastered 

his will to resist committing the crime.” §16–3–3. 

New 

Mexico 

“In order to support a verdict of insanity under the 
 

 

 

 

 

BREYER, J., dissenting 

28 KAHLER v. KANSAS 

Appendix to the opinion of BREYER, J. 

State Text 

M’Naghten test, the jury must be satisfied that the defendant (1) did not know the nature and quality of the act 

or (2) did not know that it was wrong. This rule prevailed 

in New Mexico until 1954 when this court in State v. 

White, 56 N. M. 324, 270 P. 2d 727 (1954) made a careful

analysis of the authorities and made a limited extension 

of the M’Naghten rule, adding a third ingredient. The 

court held that if the accused, (3) as a result of disease of

the mind ‘was incapable of preventing himself from committing’ the crime, he could be adjudged insane and 

thereby relieved of legal responsibility for what would

otherwise be a criminal act.” State v. Hartley, 90 N. M. 

488, 490, 565 P. 2d 658, 660 (1977). 

Virginia “As applied in Virginia, the defense of insanity provides 

that a defendant may prove that at the time of the commission of the act, he was suffering from a mental disease

or defect such that he did not know the nature and quality

of the act he was doing, or, if he did know it, he did not 

know what he was doing was wrong. . . . In addition, we

have approved in appropriate cases the granting of an instruction defining an ‘irresistible impulse’ as a form of legal insanity. The irresistible impulse doctrine is applicable only to that class of cases where the accused is able to

understand the nature and consequences of his act and 

knows it is wrong, but his mind has become so impaired

by disease that he is totally deprived of the mental power 

to control or restrain his act.” Orndorff v. Commonwealth, 279 Va. 597, 601, n. 5, 691 S. E. 2d 177, 179, n. 5 

(2010) (internal quotation marks and citations omitted). 

Moral incapacity

State Text 

Arizona “A person may be found guilty except insane if at the 

time of the commission of the criminal act the person 

was afflicted with a mental disease or defect of such severity that the person did not know the criminal act was

wrong.” Ariz. Rev. Stat. Ann. §13–502(A) (2010). 

Delaware “In any prosecution for an offense, it is an affirmative

defense that, at the time of the conduct charged, as a 

result of mental illness or serious mental disorder, the

accused lacked substantial capacity to appreciate the 

wrongfulness of the accused’s conduct.” Del. Code Ann., 

Tit. 11, §401(a) (2015). 

Illinois “A person is not criminally responsible for conduct if 

at the time of such conduct, as a result of mental disease 
 

 

 

BREYER, J., dissenting 

Cite as: 589 U. S. ____ (2020) 29 

Appendix to the opinion of BREYER, J. 

State Text 

or mental defect, he lacks substantial capacity to appreciate the criminality of his conduct.” Ill. Comp. Stat., ch. 

720, §5/6–2(a) (West 2017). 

Indiana “A person is not responsible for having engaged in

prohibited conduct if, as a result of mental disease or defect, he was unable to appreciate the wrongfulness of the

conduct at the time of the offense.” Ind. Code §35–41– 

3–6(a) (2019). 

Louisiana “If the circumstances indicate that because of a mental disease or mental defect the offender was incapable

of distinguishing between right and wrong with reference to the conduct in question, the offender shall be exempt from criminal responsibility.” La. Rev. Stat. Ann. 

§14:14 (West 2016). 

Maine “A defendant is not criminally responsible by reason 

of insanity if, at the time of the criminal conduct, as a 

result of mental disease or defect, the defendant lacked 

substantial capacity to appreciate the wrongfulness of

the criminal conduct.” Me. Rev. Stat. Ann., Tit. 17, 

§39(1) (2006). 

Ohio “A person is ‘not guilty by reason of insanity’ relative 

to a charge of an offense only if the person proves, in the 

manner specified in section 2901.05 of the Revised Code, 

that at the time of the commission of the offense, the person did not know, as a result of a severe mental disease

or defect, the wrongfulness of the person’s acts.” Ohio 

Rev. Code Ann. §2901.01(14) (Lexis 2014). 

South “It is an affirmative defense to a prosecution for a 

Carolina crime that, at the time of the commission of the act constituting the offense, the defendant, as a result of mental 

disease or defect, lacked the capacity to distinguish 

moral or legal right from moral or legal wrong or to recognize the particular act charged as morally or legally 

wrong.” S. C. Code Ann. §17–24–10(A) (2014). 

South “ ‘Insanity,’ the condition of a person temporarily or 

Dakota partially deprived of reason, upon proof that at the time

of committing the act, the person was incapable of knowing its wrongfulness, but not including an abnormality 

manifested only by repeated unlawful or antisocial behavior.” S. D. Codified Laws §22–1–2(20) (2017). 

“Insanity is an affirmative defense to a prosecution 

for any criminal offense.” §22–5–10. 

Texas “It is an affirmative defense to prosecution that, at the

time of the conduct charged, the actor, as a result of severe

mental disease or defect, did not know that his conduct 

was wrong.” Tex. Penal Code Ann. §8.01(a) (West 2011). 
 

 

 

 

 

 

 

BREYER, J., dissenting 

30 KAHLER v. KANSAS 

Appendix to the opinion of BREYER, J. 

Model Penal Code 

State Text 

Arkansas “ ‘Lack of criminal responsibility’ means that due to a 

mental disease or defect a defendant lacked the capacity

at the time of the alleged offense to either:

“(A) Appreciate the criminality of his or her conduct; 

or 

“(B) Conform his or her conduct to the requirements of

the law.” Ark. Code Ann. §5–2–301(6) (Supp. 2019). 

Connecti- “In any prosecution for an offense, it shall be an affirmcut ative defense that the defendant, at the time he committed the proscribed act or acts, lacked substantial capacity, as a result of mental disease or defect, either to

appreciate the wrongfulness of his conduct or to control

his conduct within the requirements of the law.” Conn. 

Gen. Stat. §53a–13(a) (2017). 

Hawaii “A person is not responsible, under this Code, for conduct if at the time of the conduct as a result of physical

or mental disease, disorder, or defect the person lacks 

substantial capacity either to appreciate the wrongfulness of the person’s conduct or to conform the person’s 

conduct to the requirements of law.” Haw. Rev. Stat. 

§704–400(1) (2014). 

Kentucky “A person is not responsible for criminal conduct if at

the time of such conduct, as a result of mental illness or 

intellectual disability, he lacks substantial capacity either

to appreciate the criminality of his conduct or to conform 

his conduct to the requirements of law.” Ky. Rev. Stat. 

Ann. §504.020(1) (West 2016). 

Maryland “A defendant is not criminally responsible for criminal 

conduct if, at the time of that conduct, the defendant, 

because of a mental disorder or mental retardation, 

lacks substantial capacity to:

“(1) appreciate the criminality of that conduct; or 

“(2) conform that conduct to the requirements of law.”

Md. Crim. Proc. Code Ann. §3–109(a) (2018). 

Massachusetts “1. Criminal responsibility. Where a defendant asserts a defense of lack of criminal responsibility and 

there is evidence at trial that, viewed in the light most 

favorable to the defendant, would permit a reasonable

finder of fact to have a reasonable doubt whether the defendant was criminally responsible at the time of the offense, the Commonwealth bears the burden of proving 

beyond a reasonable doubt that the defendant was crim-
 

 

BREYER, J., dissenting 

Cite as: 589 U. S. ____ (2020) 31 

Appendix to the opinion of BREYER, J. 

State Text 

inally responsible. In this process, we require the Commonwealth to prove negatives beyond a reasonable 

doubt: that the defendant did not have a mental disease 

or defect at the time of the crime and, if that is not disproved beyond a reasonable doubt, that no mental disease or defect caused the defendant to lack substantial 

capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of 

law.” Commonwealth v. Lawson, 475 Mass. 806, 811, 62

N. E. 3d 22, 28 (2016) (internal quotation marks and citation omitted). 

Michigan “It is an affirmative defense to a prosecution for a 

criminal offense that the defendant was legally insane 

when he or she committed the acts constituting the offense. An individual is legally insane if, as a result of

mental illness as defined in section 400 of the mental 

health code . . . that person lacks substantial capacity

either to appreciate the nature and quality or the wrongfulness of his or her conduct or to conform his or her conduct to the requirements of the law.” Mich. Comp. Laws

Ann. §768.21a(1) (West 2000). 

Oregon “A person is guilty except for insanity if, as a result of a 

qualifying mental disorder at the time of engaging in 

criminal conduct, the person lacks substantial capacity

either to appreciate the criminality of the conduct or to

conform the conduct to the requirements of law.” Ore. 

Rev. Stat. §161.295(1) (2019). 

Rhode “A person is not responsible for criminal conduct if at the

Island time of such conduct, as a result of mental disease or defect, his capacity either to appreciate the wrongfulness 

or his conduct or to conform his conduct to the requirements of the law were so substantially impaired that he 

cannot justly be held responsible.” State v. Carpio, 43 A.

3d 1, 12, n. 10 (R. I. 2012) (internal quotation marks 

omitted).

Vermont “The test when used as a defense in criminal cases 

shall be as follows: 

“(1) A person is not responsible for criminal conduct if 

at the time of such conduct as a result of mental disease 

or defect he or she lacks adequate capacity either to appreciate the criminality of his or her conduct or to conform his or her conduct to the requirements of law.” Vt. 

Stat. Ann., Tit. 13, §4801(a) (2019). 

West 

Virginia 

“When a defendant in a criminal case raises the issue of 

insanity, the test of his responsibility for his act is 

whether, at the time of the commission of the act, it was 
 

 

BREYER, J., dissenting 

32 KAHLER v. KANSAS 

Appendix to the opinion of BREYER, J. 

State Text 

the result of a mental disease or defect causing the

accused to lack the capacity either to appreciate the 

wrongfulness of his act or to conform his act to the requirements of the law.” State v. Fleming, 237 W. Va. 44,

52–53, 784 S. E. 2d 743, 751–752 (2016). 

Wisconsin “A person is not responsible for criminal conduct if at the 

time of such conduct as a result of mental disease or defect the person lacked substantial capacity either to appreciate the wrongfulness of his or her conduct or conform his or her conduct to the requirements of law.” Wis. 

Stat. §971.15(1) (2016). 

Wyoming “A person is not responsible for criminal conduct if at

the time of the criminal conduct, as a result of mental

illness or deficiency, he lacked substantial capacity either to appreciate the wrongfulness of his conduct or to 

conform his conduct to the requirements of law.” Wyo.

Stat. Ann. §7–11–304(a) (2019). 

District of “A person is not responsible for criminal conduct if at the

Columbia time of such conduct as a result of a mental disease or 

defect he lacked substantial capacity either to recognize 

the wrongfulness of his conduct or to conform his conduct to the requirements of the law.” Bethea v. United 

States, 365 A. 2d 64, 79, n. 30 (D. C. 1976). 

Unique formulation

State Text 

New 

Hampshire 

“A defendant asserting an insanity defense must prove 

two elements: first, that at the time he acted, he was suffering from a mental disease or defect; and, second, that 

a mental disease or defect caused his actions.” State v. 

Fichera, 153 N. H. 588, 593, 903 A. 2d 1030, 1034 (2006). 

North “An individual is not criminally responsible for criminal

Dakota conduct if, as a result of mental disease or defect existing 

at the time the conduct occurs: 

“a. The individual lacks substantial capacity to comprehend the harmful nature or consequences of the conduct, or the conduct is the result of a loss or serious distortion of the individual’s capacity to recognize reality; 

and 

“b. It is an essential element of the crime charged that

the individual act willfully.” N. D. Cent. Code Ann. 

§12.1–04.1–01(1) (2012).