Diminished Capacity

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 of—cannot 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.