Diminished Capacity

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 [Page 218]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.

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