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Diminished Capacity


Legal guilt or culpability for the commission of a crime requires both that the prosecution must prove beyond a reasonable doubt all the definitional elements of the crime charged, including the mental statethe mens rea required by the crime's definitionand that an affirmative defense, such as the excuse of legal insanity or duress, is not established.

Diminished capacity refers to two distinct doctrines: the use of evidence of mental abnormality to negate a mens rea required by the definition of the crime charged (the mens rea variant ) and the use of mental abnormality evidence to establish some type of partial affirmative defense of excuse (the partial excuse variant ). Courts have used various other terms, such as diminished responsibility, to refer to one or both of these distinct doctrines, but the term used is unimportant. Confusion arises, however, when the two types of doctrine are not clearly distinguished. Neither entails the other and distinct legal and policy concerns apply to each. After a brief description of the reasons these doctrines developed, this essay will address both variants.

Historical background

Prior to the development of either variant of diminished capacity, a defendant suffering from mental abnormality had limited ability to use such abnormality to avoid conviction either by negating the prosecution's prima facie case or by establishing an affirmative defense.

Mens rea variant. Mental abnormality can potentially negate mens rea primarily in cases in which the abnormality is quite severe and produces a cognitive mistake. For example, the commentary to the Model Penal Code uses the example of a hallucinating defendant who strangles a victim believing that he or she is squeezing a lemon rather than a person's throat. This is a famous but silly example because such a hallucination is extremely improbable, but if it were true, the defendant did not form in fact the mental state, the intent to kill another, required by the definition of intentional homicide.

Historically, the difficulty for negating mens rea was that traditional doctrine required that mistakes had to be objectively reasonable, and a mistake mental abnormality produces is definitionally unreasonable, even if subjectively mens rea does not exist. Thus, evidence of such mistakes was excluded, even though it is logically relevant to whether a requisite mens rea was in fact present. This result seemed unfair to many courts because a defendant subjectively lacking the requisite mens rea for reasons not the agent's fault does not appear culpable for the crime charged. Consequently, many courts began to permit the admission of mental abnormality evidence to negate mens rea, although for reasons to be explored below, they usually substantially restricted such use of abnormality evidence. But there is a constant and continuing tension in criminal law between objective and subjective approaches to culpability, and many courts continued to exclude such evidence, often because they confused the use of such evidence to negate the prosecution's prima facie case with the entirely distinct, full excuse of legal insanity or because they distrusted mental abnormality evidence altogether.

Partial excuse variant. The affirmative defense of legal insanity, which was traditionally doctrinally limited and distrusted or feared by juries and judges alike, provided the only means to introduce mental abnormality evidence and few defendants could expect to succeed with this defense, even if the mental disorder was obvious and severe. Some criminal defendants patently suffered from a mental abnormality insufficient to support a successful insanity defense, but that substantially compromised their capacity for rationality. Such defendants might have the mens rea required by the definition of the crime and might be criminally responsible, but they seemed less criminally responsible than defendants without abnormality who were charged with the same crime. Nonetheless, with the very limited exception of the provocation/passion doctrine that reduced an intentional killing from murder to the lesser crime of voluntary manslaughter and that, anyway, was not understood as a mental abnormality doctrine, no doctrine provided a partial excuse to legally responsible defendants whose responsibility was diminished by mental abnormality. For example, a mentally abnormal defendant who killed intentionally and with premeditation had no doctrinal tool to avoid conviction and punishment for the most culpable degree of crimefirst-degree murdereven if the killing was the highly irrational product of substantial mental abnormality.

Failure to consider partially excusing mental abnormality evidence also seemed unfair and some courts and legislatures tried to create means to permit partial excusing claims. But the tension between subjective and objective guilt applied in the context of partial excuse much as it did in the context of mens rea negation. Consequently, both courts and legislatures feared creating a subjective, generic partial excuse, again, in part, because they lacked confidence in the reliability of evidence of mental abnormality. Furthermore, courts were hindered because creating a genuine partial excuse appears to be a "legislative act" that goes beyond a court's prerogative. In a few jurisdictions, courts tried to develop a partial excuse in the guise of adopting the mens rea variant, but these attempts used extremely problematic mens rea concepts and were confusing. As will be discussed below, legislatures and courts permitted some partial excusing claims, but no generic partial excuse for mental abnormality exists in any jurisdiction in the United States or in English law.

The mens rea variant

The logic of the mens rea variant is impeccable. Crimes are defined by their elements and the prosecution must prove all these elements beyond a reasonable doubt to secure a conviction. If the prosecution is unable to prove an element, either because its case is weak or because the defendant has sufficient evidence to cast reasonable doubt on the presence of an element of the prosecution's case, then the defendant should be acquitted of a crime requiring that element. The defendant using the mens rea variant of diminished capacity seeks simply to use evidence of mental abnormality to cast reasonable doubt on the presence of a mental state element that is part of the definition of the crime charged. Such use of mental abnormality evidence is not a full or partial affirmative defense. Use of mental abnormality evidence to deny the prosecution's prima facie case is functionally and doctrinally indistinguishable from the use of any other kind of evidence for the same purpose, and it thus does not warrant a special name, as if it were a unique doctrine. The general question of whether defendants ought to be allowed to negate mens rea using probative evidence is different from the specific evidentiary question of whether mental abnormality evidence is more or less reliable than other evidence used to negate mens rea.

Justice or fairness seems to require permitting a criminal defendant to use relevant evidence to cast reasonable doubt on the prosecution's case when criminal punishment and stigma are at stake. Indeed, it would be unconstitutional as a denial of various rights, such as the confrontation clause, completely to prohibit introduction of all such evidence. Nonetheless, a criminal defendant's right to introduce relevant evidence may be denied for good reason and the U.S. Supreme Court has never held that the Constitution requires the admission of mental abnormality evidence to negate mens rea. About half the American jurisdictions exclude mental abnormality evidence altogether when it is offered to negate mens rea and the other half permit introduction but typically place substantial restrictions on the use of the evidence.

Total exclusion of mental abnormality evidence. The most common reasons used to justify total exclusion of mental abnormality evidence to negate mens rea are that the use of evidence to negate mens rea is mistakenly understood as an affirmative defense, that mental abnormality evidence is considered particularly unreliable in general or for this purpose, and that permitting the use of such evidence would compromise public safety unduly. If mens rea negation is wrongly thought to be an affirmative defense, it may appear redundant with the defense of legal insanity, or a court might believe that creating a new affirmative defense is the legislature's prerogative. These might be good reasons to reject admission of mental abnormality evidence, if mens rea negation were an affirmative defense. But these reasons are unpersuasive because they rest on an entirely confused doctrinal foundation.

The unreliability rationale for total exclusion is stronger in principle because courts are always free to reject arguably relevant evidence if it is unreliable or confusing. The difficulty with this rationale for total exclusion is that mental abnormality evidence is routinely considered sufficiently reliable and probative to be admitted in an enormous array of criminal and civil law contexts, including competence to stand trial, legal insanity, competence to contract, and others. Exclusion of such evidence offered to defeat the prosecution's case in a criminal case appears unfair. Because the defendant's liberty and reputation are threatened by the power of the state, criminal defendants are afforded special protections in our adversary system, such as the right to have the prosecution prove its case by the rigorous beyond-a-reasonable-doubt standard. For the same reason, there is also powerful reason to provide defendants special latitude to admit potentially exculpatory evidence, especially when it is admitted in other contexts where much less is at stake. It seems especially unfair to exclude evidence of mental abnormality, which is rarely if ever the defendant's fault, when most jurisdictions routinely admit evidence of voluntary intoxication, which is typically the defendant's fault, to negate some mens rea.

The public safety rationale is also sound in principle. Opponents of admission fear that if a mentally abnormal and dangerous defendant uses abnormality evidence successfully to negate all mens rea, outright acquittal and release of a dangerous agent will result. Virtually automatic involuntary civil commitment follows a successful affirmative defense of legal insanity, but the state has less effective means to preventively confine dangerous defendants acquitted outright.

The problem with the public safety rationale is practical rather than theoretical. Mental disorders, including those that are most severe and compromise contact with reality, may cause agents to have crazy reasons to do what they do, but they seldom prevent people from forming intentions to act, from having the narrow types of knowledge required by legal mens rea, and the like. Consequently, very few defendants with mental disorder will be able to gain outright acquittal by negating all mens rea or even to reduce their conviction by negating some mens rea. Moreover, the mens rea termed "negligence"failure to be aware of a risk that one has created and should be aware ofcannot be negated by mental abnormality because such failure is per se objectively unreasonable.

The only possible exception to the observation that mental abnormality seldom negates mens rea is the mental state of premeditation required by many jurisdictions for conviction for intentional murder in the first degree. On occasion, a person with a disorder may kill on the spur of the moment motivated by a command hallucination or a delusional belief. Such people are capable of premeditating, but the mental abnormality evidence simply tends to show that they did not premeditate in fact on this occasion. Mental disorder thus rarely negates mens rea and permitting defendants to introduce such evidence in the small number of cases in which it might do so would not substantially compromise public safety.

In sum, all the rationales for total exclusion of mental abnormality evidence proffered to negate mens rea are flawed and the criminal defendant's interest in admission of such evidence is strong.

Limited admission of mental abnormality evidence. Many courts have recognized the fairness rationale for admission of mental abnormality evidence to negate mens rea, including awareness of the analogy to the admission of voluntary intoxication evidence. As a logical matter, the evidence should be admitted to negate any mens rea that might have been negated in fact and, indeed, this is the Model Penal Code position. Nonetheless, virtually all jurisdictions that have permitted using mental abnormality evidence to negate mens rea have placed substantial limitations on doing so, largely because they fear the outright acquittal that in principle could result from following the pure logical relevance standard for admission. The logic of limited admission is thus the logic of a policy compromise between considerations of fairness and public safety: A defendant is able to negate some but not all mens rea, which typically results in conviction for a lesser offense than the crime charged. The effect of mental abnormality on culpability is thus considered, albeit partially, and a potentially dangerous defendant does not go free entirely, albeit the sentence is abbreviated.

Courts have tried to justify the particular limitations they place on the admission of mental abnormality evidence to negate mens rea, but other than the consequence of avoiding outright acquittal, there is no particular logic to the various limitations. Some jurisdictions permit only the negation of premeditation, although other mens reas may be negated in fact; others draw the confusing, technical distinction between specific and general intent and permit negation only of the former, even if the latter is in fact negated. Nonetheless, even restricted admissibility provides defendants with some opportunity to mitigate punishment and stigma.

The partial excuse variant

The logic of the partial responsibility variant is also impeccable. In general, the capacity for rationality, the capacity to grasp and be guided by reason, is the touchstone of moral and legal responsibility for one's actions. Mental abnormality potentially compromises moral and legal responsibility because in some cases it renders the defendant so irrational that the defendant is not a responsible agent. In the case of legal insanity, for example, mental disorder must be present as a cause of sufficient irrationality, but it is the irrationality and not the disorder per se that is the genuine basis of the excuse. The capacity for rationality, which is the basis of responsibility, is a continuum, however, and in principle responsibility should also be a continuum. The complete excuse of legal insanity does not contain degrees, but many mentally abnormal defendants who do not meet the test for legal insanity may nonetheless suffer from serious rationality impairments that compromise their responsibility and that thus appear to require a partial excuse.

Notwithstanding the logic, no generic partial excuse variant of diminished capacity that would apply to all crimes exists in any jurisdiction in the United States or in England. Courts are unwilling to create a generic excuse for many reasons, including the belief that they do not have the power to create new excuses, the fear that they will be inundated with potentially confusing or unjustified claims, and the fear that dangerous defendants might go free earlier than concerns for public safety require. Legislatures appear unwilling to enact a generic partial excuse because, in general, legislatures are not responsive to claims that are to the advantage of wrongdoers.

Partial excusing doctrines and practices. Despite reluctance to adopt a partial excuse, courts and legislatures have adopted various doctrines or practices that are in fact forms of partial excuse. Most prominent are (1) the Model Penal Code's "extreme emotional disturbance" doctrine (sec. 210.3.1(b)) and English "diminished responsibility," both of which reduce a conviction of murder to the lesser crime of manslaughter; (2) the use of mental abnormality evidence as a mitigating factor at sentencing hearings; and, (3) one interpretation of the common law provocation/passion doctrine, which reduces an intentional killing from murder to voluntary manslaughter.

The extreme emotional disturbance doctrine, promulgated by the Model Penal Code and adopted in a small minority of American states, reduces murder to manslaughter if the killing occurred when the defendant was in a state of extreme mental or emotional disturbance for which there was reasonable explanation or excuse. Mental abnormality evidence is admissible in most jurisdictions to establish that such disturbance existed. English diminished responsibility permits the reduction to manslaughter if the defendant killed in a state of substantially impaired mental responsibility arising from mental abnormality. Neither doctrine negates the lack of intent or conscious awareness of a very great risk of death that is required for the prosecution to prove murder. Both simply reduce the degree of conviction and thus punishment and stigma because mental abnormality diminishes culpability.

The language of these doctrines is sufficiently general to apply to any crime as an affirmative defense, but this is not the law. Both doctrines exist only within the law of homicide, but in principle both operate and could be formally treated as generic affirmative defenses of partial excuse because nothing in the language of either doctrine entails that it applies only to homicide. Indeed, in some jurisdictions, the extreme emotional disturbance doctrine is explicitly treated as an affirmative defense, but only to homicide, and the defendant bears the burden of persuasion to establish the defense.

Many jurisdictions in the United States and English law also contain the provocation/passion doctrine, which reduces a murder to manslaughter if the defendant killed subjectively in the "heat of passion" in immediate response to a "legally adequate" or "objective" provocation, that is, a provoking event, such as finding one's spouse in the act of adultery, that would create an inflamed psychological state in a reasonable person. The defendant kills intentionally and is criminally responsible, but the provocation/passion doctrine reduces the degree of blame and punishment. The rationale supporting this mitigating doctrine is controversial, but one interpretation is that psychological states such as "heat of passion" diminish rationality and responsibility and the defendant is not fully at fault for being in such a diminished condition because the provocation was sufficient to put even a reasonable person in such a state. On this interpretation, the provocation/passion doctrine is a form of partial excuse related to but narrower than extreme emotional disturbance and diminished responsibility. Indeed, the extreme emotional disturbance doctrine was created to respond to the same moral concerns about responsibility as provocation/passion, but also to expand the class of defendants to whom these moral concerns apply and to whom the benefit of a partial mitigating condition should be provided.

In jurisdictions that give judges unguided or guided sentencing discretion, mental abnormality is a factor traditionally used to argue for a reduced sentence. Many capital sentencing statutes explicitly mention mental abnormality as a mitigating condition and some even use the language of the insanity defense or the extreme emotional disturbance doctrine as the mitigation standard. The partial excuse logic of such sentencing practices is conceded and straightforward. A criminally responsible defendant whose behavior satisfied all the elements of the offense charged, including the mens rea, and who has no affirmative defense, may nonetheless be less responsible because mental abnormality substantially impaired the defendant's rationality.

Confusion with the mens rea variant. A small number of courts have implicitly adopted a form of partial excuse by re-interpreting mens rea elements. For example, the influential California Supreme Court interpreted the elements of murder highly atypically to include not only the intent to kill, but also the requirement that the defendant comprehend his or her duty to govern actions in accord with the law. A defendant lacking such comprehension as a result of mental abnormality could not be guilty of murder, even if the defendant killed intentionally and there was no provocation. It was this interpretation of the elements of homicide that allowed a jury to find that the killer in the famous "Twinkies" case, Dan White, could be guilty only of manslaughter because ingestion of junk food allegedly affected his mental state and in part contributed to depriving him of the necessary comprehension for murder. It is apparent that such "comprehension" is in fact a partial excuse doctrine, rather than a traditional mental state element requirement for murder. Indeed, it seems conceptually and operationally indistinguishable from the extreme mental or emotional disturbance doctrine or from the language of some insanity defense tests. Such indirect and confusing means to establish partial excusing doctrines met with intense criticism and have been abolished in virtually all jurisdictions, including California.

A generic partial excuse?

Although no jurisdiction in the United States and English law contains a formal, generic partial excuse, the theoretical and moral case for such an excuse is strong. Indeed, the moral logic of the excuse is conceded by sentencing practices that reduce sentences because the offender suffered from substantial mental abnormality. Mental abnormality can substantially compromise rationality and responsibility among offenders who are not legally insane, including many who suffer from newly discovered syndromes, but such offenders now have only limited and often entirely discretionary means to make such claims. Consequently, many offenders may be blamed and punished more than they deserve. Creating a partial excuse would create many practical problems, including establishing the standard, insuring that the courts are not inundated with unmeritorious claims, and devising appropriate dispositions for offenders who are partially excused. But if the case for a partial excuse is sufficiently strong, as it seems to be, justice requires that our criminal law should try to develop such a formal doctrine and should not treat such questions in an unduly limited or discretionary manner.

Stephen J. Morse

See also Excuse: Theory; Excuse: Insanity; Excuse: Intoxication; Mens Rea; Mentally Disordered Offenders.


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People v. White, 172 Cal. Rptr. 612 (App. 1981).

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Diminished Capacity


This doctrine recognizes that although, at the time the offense was committed, an accused was not suffering from a mental disease or defect sufficient to exonerate him or her from all criminal responsibility, the accused's mental capacity may have been diminished by intoxication, trauma, or mental disease so that he or she did not possess the specific mental state or intent essential to the particular offense charged.

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"Diminished Capacity." West's Encyclopedia of American Law. . 21 Aug. 2017 <>.

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