diminished capacity a legal defense to criminal liability that exists in two distinct forms: (1) the mens rea variant, in which a defendant uses evidence of mental abnormality to cast doubt on the prosecution’s assertion that, at the time of the crime, the defendant possessed the mental state criteria, the mens rea, required by the legal definition of the offense charged; and (2) the partial responsibility variant, in which a defendant uses evidence of mental abnormality to support a claim that, even if the defendant’s mental state satisfied the mens rea criteria for the offense, the defendant’s responsibility for the crime is diminished and thus the defendant should be convicted of a lesser crime and/or a lesser sentence should be imposed. The mental abnormality may be produced by mental disorder, intoxication, trauma, or other causes. The mens rea variant is not a distinct excuse: a defendant is simply arguing that the prosecution cannot prove the definitional, mental state criteria for the crime. Partial responsibility is an excuse, but unlike the similar, complete excuse of legal insanity, partial responsibility does not produce total acquittal; rather, a defendant’s claim is for reduced punishment. A defendant may raise either or both variants of diminished capacity and the insanity defense in the same case. For example, a common definition of firstdegree murder requires the prosecution to prove that a defendant intended to kill and did so after premeditation. A defendant charged with this crime might raise both variants as follows. To deny the allegation of premeditation, a defendant might claim that the killing occurred instantaneously in response to a ‘command hallucination.’ If believed, a defendant cannot be convicted of premeditated homicide, but can be convicted of the lesser crime of second-degree murder, which typically requires only intent. And even a defendant who killed intentionally and premeditatedly might claim partial responsibility because the psychotic mental state rendered the agent’s reasons for action nonculpably irrational. In this case, either the degree of crime might be reduced by operation of the partial excuse, rather than by negation of definitional mens rea, or a defendant might be convicted of first-degree murder but given a lesser penalty. In the United States the mens rea variant exists in about half the jurisdictions, although its scope is usually limited in various ways, primarily to avoid a defendant’s being acquitted and freed if mental abnormality negated all the definitional mental state criteria of the crime charged. In English law, the mens rea variant exists but is limited by the type of evidence usable to support it. No American jurisdiction has adopted a distinct, straightforward partial responsibility variant, but various analogous doctrines and procedures are widely accepted. For example, partial responsibility grounds both the doctrine that intentional killing should be reduced from murder to voluntary manslaughter if a defendant acted ‘in the heat of passion’ upon legally adequate provocation, and the sentencing judge’s discretion to award a decreased sentence based on a defendant’s mental abnormality. In addition to such partial responsibility analogues, England, Wales, and Scotland have directly adopted the partial responsibility variant, termed ‘diminished responsibility,’ but it applies only to prosecutions for murder. ‘Diminished responsibility’ reduces a conviction to a lesser crime, such as manslaughter or culpable homicide, for behavior that would otherwise constitute murder.
See also FREE WILL PROBLEM , MENS REA, PHILOSOPHY OF LAW. S.J.M.