Diminished Capacity

Prior to the development of either variant of diminished capacity, a defendant suffering from mental abnormality had limited ability to use such abnormality to avoid conviction either by negating the prosecution's prima facie case or by establishing an affirmative defense.

Mens rea variant. Mental abnormality can potentially negate mens rea primarily in cases in which the abnormality is quite severe and produces a cognitive mistake. For example, the commentary to the Model Penal Code uses the example of a hallucinating defendant who strangles a victim believing that he or she is squeezing a lemon rather than a person's throat. This is a famous but silly example because such a hallucination is extremely improbable, but if it were true, the defendant did not form in fact the mental state, the intent to kill another, required by the definition of intentional homicide.

Historically, the difficulty for negating mens rea was that traditional doctrine required that mistakes had to be objectively reasonable, and a mistake mental abnormality produces is definitionally unreasonable, even if subjectively mens rea does not exist. Thus, evidence of such mistakes was excluded, even though it is logically relevant to whether a requisite mens rea was in fact present. This result seemed unfair to many courts because a defendant subjectively lacking the requisite mens rea for reasons not the agent's fault does not appear culpable for the crime charged. Consequently, many courts began to permit the admission of mental abnormality evidence to negate mens rea, although for reasons to be explored below, they usually substantially restricted such use of abnormality evidence. But there is a constant and continuing tension in criminal law between objective and subjective approaches to culpability, and many courts continued to exclude such evidence, often because they confused the use of such evidence to negate the prosecution's prima facie case with the entirely distinct, full excuse of legal insanity or because they distrusted mental abnormality evidence altogether.

Partial excuse variant. The affirmative defense of legal insanity, which was traditionally doctrinally limited and distrusted or feared by juries and judges alike, provided the only means to introduce mental abnormality evidence and few defendants could expect to succeed with this defense, even if the mental disorder was obvious and severe. Some criminal defendants patently suffered from a mental abnormality insufficient to support a successful insanity defense, but that substantially compromised their capacity for rationality. Such defendants might have the mens rea required by the definition of the crime and might be criminally responsible, but they seemed less criminally responsible than defendants without abnormality who were charged with the same crime. Nonetheless, with the very limited exception of the provocation/passion doctrine that reduced an intentional killing from murder to the lesser crime of voluntary manslaughter and that, anyway, was not understood as a mental abnormality doctrine, no doctrine provided a partial excuse to legally responsible defendants whose responsibility was diminished by mental abnormality. For example, a mentally abnormal defendant who killed intentionally and with premeditation had no doctrinal tool to avoid conviction and punishment for the most culpable degree of crime—first-degree murder—even if the killing was the highly irrational product of substantial mental abnormality.

Failure to consider partially excusing mental abnormality evidence also seemed unfair and some courts and legislatures tried to create means to permit partial excusing claims. But the tension between subjective and objective guilt applied in the context of partial excuse much as it did in the context of mens rea negation. Consequently, both courts and legislatures feared creating a subjective, generic partial excuse, again, in part, because they lacked confidence in the reliability of evidence of mental abnormality. Furthermore, courts were hindered because creating a genuine partial excuse appears to be a "legislative act" that goes beyond a court's prerogative. In a few jurisdictions, courts tried to develop a partial excuse in the guise of adopting the mens rea variant, but these attempts used extremely problematic mens rea concepts and were confusing. As will be discussed below, legislatures and courts permitted some partial excusing claims, but no generic partial excuse for mental abnormality exists in any jurisdiction in the United States or in English law.