Diminished Capacity

Diminished capacity refers to two distinct doctrines. The first, known as the mens rea variant, refers to the use of evidence of mental abnormality to negate a mens rea—a mental state such as intent, required by the definition of the crime charged (the mens rea variant). The second, known as the partial responsibility variant, refers to the use of mental abnormality evidence to establish some type of partial affirmative defense of excuse. 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.

The Mens Rea Variant

Mental abnormality can negate mens rea, primarily in cases in which the disorder is quite severe and produces a cognitive mistake. For example, in Clark v. Arizona (2006), a recent case that reached the Supreme Court of the United States, the defendant claimed that he believed that the police officer he killed was really a space alien impersonating a police officer. If this was true, the defendant did not intend to kill a human being with the knowledge that the victim was a police officer. Historically, the legal objection to using mental abnormality to negate mens rea was that traditional doctrine required that mistakes had to be objectively reasonable and a mistake that mental abnormality produces is definitionally unreasonable. Thus, evidence of such mistakes was excluded, even though it is logically relevant to whether a requisite mens rea was in fact present.

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. If the prosecution is unable to prove an element, 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 a partial affirmative defense. It 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.

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. Nonetheless, a criminal defendant’s right to introduce relevant evidence may be denied for good reason, and the U.S. Supreme Court recently held that the Constitution does not require the admission of most kinds of mental abnormality evidence offered to negate mens rea, even if such evidence is logically relevant and probative. About half the American jurisdictions exclude mental abnormality evidence altogether when it is offered to negate mens rea, and the other half permit its introduction but typically place substantial restrictions on the use of the evidence.

Total Exclusion of Mental Abnormality Evidence

The most common justifications for exclusion of mental abnormality evidence to negate mens rea are that courts and legislatures confuse the mens rea claim with a partial or complete 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. 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. If mens rea negation were an affirmative defense, these might be good reasons to reject the admission of mental abnormality evidence, but these reasons are unpersuasive because they rest on a confused doctrinal foundation.

The unreliability rationale for exclusion is stronger in principle because courts are always free to reject unreliable evidence. The difficulty with this rationale is that mental abnormality evidence is routinely considered sufficiently reliable and probative to be admitted in a wide array of criminal and civil law contexts, including competence to stand trial, legal insanity, competence to contract, and others. Criminal defendants are afforded special protections in our adversary system because the defendant’s liberty and reputation are threatened by the power of the State. For the same reason, there is also a powerful motivation to provide defendants special latitude to admit potentially exculpatory evidence, especially when evidence of the same type 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 in some circumstances routinely admit evidence of voluntary intoxication to negate mens rea.

The public safety rationale is also sound in principle. 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 may cause agents to have profoundly irrational reasons for action, 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. Moreover, the mens rea termed negligence—unreasonable failure to be aware of an unjustifiable risk that one has created—cannot be negated by mental abnormality because such failure is per se objectively unreasonable. Consequently, very few defendants with mental disorder will be able to gain outright acquittal by negating all mens rea or will even be able to reduce their conviction by negating some mens rea. Public safety would not be compromised by the mens rea variant.

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. And even if premeditation is negated, the intent to kill is not.

Limited Admission of Mental Abnormality Evidence

If the rationale for the mens rea variant is accepted, as a logical matter, the evidence should be admitted to negate any mens rea that might have been negated in fact. 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 incorrectly fear large numbers of outright acquittals that could result from following the pure logical relevance standard for admission. Limited admission is thus based on 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. 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.

Partial Responsibility Variant

Some criminal defendants who acted with the mens rea required by the definition of the crime charged and who cannot succeed with the insanity defense nonetheless have mental abnormalities that substantially compromise their capacity for rationality. The logic of the partial responsibility variant flows from this observation. In general, the capacity for rationality, the capacity to grasp and be guided by reason, is the touchstone of moral and legal responsibility. 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. The capacity for rationality is a continuum, however, and in principle, responsibility should also be a continuum, allowing for a partial defense. Nonetheless, no generic partial excuse for diminished rationality arising from mental abnormality exists in any jurisdiction in the United States or in English law. Thus, for example, a mentally abnormal defendant who killed intentionally and with premeditation has no doctrinal tool to avoid conviction and punishment for the most culpable degree of crime— first degree murder—even if the killing was highly irrationally motivated as a result of substantial mental abnormality.

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 cases, and the fear that dangerous defendants might go free too quickly and endanger the public. Furthermore, courts believe that creating a genuine partial excuse is a “legislative act” that exceeds judicial 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, were confusing, and have largely been abandoned. 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 and because legislators, too, fear the consequences for public safety.

Partial Responsibility Doctrines and Practices

Despite their reluctance to adopt a generic partial responsibility doctrine, courts and legislatures have adopted various doctrines or practices that are in fact forms of partial excuse. Most prominent are (a) 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; (b) one interpretation of the common-law provocation/ passion doctrine, which reduces an intentional killing from murder to voluntary manslaughter; and (c) the use of mental abnormality evidence as a mitigating factor at sentencing hearings.

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 of the charge 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. These partial responsibility 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.

Many jurisdictions in the United States and in 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. In this interpretation, the provocation/passion doctrine is a form of partial excuse related to but narrower than extreme emotional disturbance and diminished responsibility.

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 is 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.


  1. American Law Institute. (1962). Model Penal Code. Philadelphia: Author.
  2. Arenella, P. (1977). The diminished capacity and diminished responsibility defenses: Two children of a doomed marriage. Columbia Law Review, 77, 827-865.
  3. Clark v. Arizona, 548 U.S._126 S. Ct. 2709 (2006).
  4. Fingarette, H., & Hasse, A. F. (1979). Mental disabilities and criminal responsibility. Berkeley: University of California Press.
  5. Horder, J. (1992). Provocation and responsibility. Oxford, UK: Clarendon Press.
  6. Mackay, R. D. (1995). Mental condition defenses in the criminal law. Oxford, UK: Clarendon Press.
  7. Morse, S. J. (1984). Undiminished confusion in diminished capacity. Journal of Criminal Law and Criminology, 75, 1-55.
  8. Morse, S. J. (1993). Diminished capacity. In S. Shute, J. Gardner, & J. Horder (Eds.), Action and value in criminal law (pp. 239-278). Oxford, UK: Clarendon Press.

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