Diminished Capacity

Legal guilt or culpability for the commission of a crime requires both that the prosecution must prove beyond a reasonable doubt all the definitional elements of the crime charged, including the mental state—the mens rea required by the crime's definition—and that an affirmative defense, such as the excuse of legal insanity or duress, is not established.

Diminished capacity refers to two distinct doctrines: the use of evidence of mental abnormality to negate a mens rea required by the definition of the crime charged (the mens rea variant) and the use of mental abnormality evidence to establish some type of partial affirmative defense of excuse (the partial excuse variant). 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. After a brief description of the reasons these doctrines developed, this essay will address both variants.