In criminal law, diminished responsibility (or diminished capacity) is a potential defense by excuse by which defendants argue that although they broke the law, they should not be held fully criminally liable for doing so, as their mental functions were "diminished" or impaired.
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Diminished capacity is a partial defense to charges that require that the defendant act with a particular state of mind. For example, if the felony murder rule does not apply, first degree murder requires that the state prove beyond a reasonable doubt that the defendant acted with premeditation, deliberation, and the specific intent to kill—all three are necessary elements of the state's case. If evidence exists, sufficient to create a reasonable doubt as to whether the defendant because of mental illness or "defect" possessed the capacity to premeditate, deliberate or form the specific intent to kill then the state cannot convict the defendant of first degree murder. This does not mean that the defendant is entitled to an acquittal. The defendant still might be convicted of second-degree murder which only requires that the defendant act with general malice.
The defense's acceptance in American jurisdictions varies considerably. The majority of states have adopted it by statute or case decision, and a minority even recognise broader defenses such as "irresistible impulse". Some U.S. states restrict the defense to the charge of murder only where a successful defense will result in a manslaughter conviction instead of murder. Until recently, the Republic of Ireland did not accept the partial defense. The Irish Supreme Court had rejected the existence of the defense in DPP v O'Mahony. The case was recently abrogated, however, by enactment of the Criminal Law (Insanity) Act 2006, effective June 1, 2006. The act, in pertinent part, specifically adopted the partial defense for the charge of murder where a successful defense will result in a manslaughter conviction instead of murder.
The defense is to be contrasted with insanity which is a complete but affirmative defense. In most jurisdictions a defendant would be acquitted on the grounds of insanity if the defendant established to the satisfaction of the jury that he suffered from such a mental disease or defect that he was unable to appreciate the consequences of his actions or did not know what he was doing was wrong. As noted a successful insanity defense will result in acquittal although a number of jurisdictions have adopted the guilty but insane verdict. The defense of insanity and diminished capacity although clearly distinct are not inconsistent defenses and both may be at issue in the same case. The critical distinctions are that diminished capacity is a partial, negating defense (negates an element of the state's case) with the burden on the state to show that the defendant acted with the requisite state of mind while insanity is a complete but affirmative defense—the defendant bearing the burden of proving that he was legally insane.
This is an aspect of a more general insanity defense (see the M'Naghten rules). The defense "was first recognized by Scottish common law to reduce the punishment of the 'partially insane'." It developed from the practice of juries in the 19th century of returning verdicts of guilty with a recommendation as to mercy or mitigation of sentence to reflect any extenuating circumstances. In a series of decisions, given mainly by Lord Deas, a doctrine grew that various types of mental weakness could have the effect of reducing what would otherwise be a conviction for murder (which attracted capital punishment) to one for culpable homicide (where the courts had greater discretion in sentencing). An example of a "diminished capacity" might be extremely low intelligence. In the English case of R v Raven, a man who had a physical age of 22 years but a mental age of only 9 years felt provoked by homosexual advances and killed his perceived attacker. His mental deficiency was not in dispute and, since a child of 9 years would not have been criminally responsible (see s50 Children and Young Persons Act 1933), and his mental responsibility for his acts was substantially impaired, manslaughter was the only realistic verdict. The rationale of the defense is that, as a precondition to punishment, the criminal law requires conduct to be voluntary. If something interferes with the capacity of the individual to choose to break the law, this should be reflected by an excuse or exculpation. The law should balance the need to be fair to the individual wrongdoer, but equally offer some protection to society from a person who may not have complete control over their behavior.
The effect of the defense varies between the jurisdictions. In some, it will result in full excuse and therefore produce a verdict of "not guilty". In others, it offers only exculpation to a degree, resulting in the substitution of a lesser offence (e.g., manslaughter instead of murder) or a mitigated sentence.