Diminished Capacity

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.