Diminished responsibility
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 criminally liable for doing so, as their mental functions were "diminished" or impaired.  The defence's acceptance in American jurisdictions vary considerably.  The majority of states have adopted it by statute or case decision, and a minority even recognise broader defences such as 'irresistible impulse.' Some American states restrict the defence to the charge of murder only where a successful defence will result in a manslaughter conviction instead of murder.   Until recently, the Republic of Ireland did not accept the partial defence.  The Irish Supreme Court had rejected the existence of the defence in The People (DPP) v Joseph O' Mahony  ILRM 244.  The case was recently abrogated, however, by enactment of the Criminal Law (Insanity) Act 2006, effective 1 June 2006.  The act, in pertinent part, specifically adopted the partial defence for the charge of murder where a successful defence will result in a manslaughter conviction instead of murder.

# Discussion
This is an aspect of a more general insanity defense (see the M'Naghten Rules). Peter Arenella, in the Columbia Law Review (1977 p.830), stated, "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 (1982) Crim. LR 51 a man who had a physical age of 22 years but a mental age of only 9 years was provoked by homosexual attacks and killed the 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.
California was the first state in the U.S. to adopt the diminished capacity defense, beginning with People v. Wells, 202 P.2d 53 (1949), and People v. Gorshen, 336 P.2d 492 (1959). The doctrine would soon be abolished by ballot initiative in 1982 following the negative publicity surrounding the case of Dan White, who had killed George Moscone and Harvey Milk.  While White's defense team did argue successfully for a ruling of diminished capacity, resulting in a verdict of voluntary manslaughter rather than murder, an urban legend that the defense had blamed White's actions on the ingestion of sugar and junk food (the so-called "Twinkie defense") sprang up out of inaccurate media coverage.  One participant in the debate over diminished capacity rulings waved a Twinkie in the air to make his point.  Currently, the California Penal Code states (2002), "The defense of diminished capacity is hereby abolished ... there shall be no defense of diminished capacity, diminished responsibility, or irresistible impulse..."

# English law

# Scottish law
During the course of the 20th century the courts began to limit the mental conditions falling within diminished responsibility. In HM Advocate v Savage (1923) JC 49 Lord Alness addressed the jury (at 51):
This statement became the authoritative version of the test for diminished responsibility and the various factors mentioned by Lord Alness were regarded as being cumulative in nature. The effect was that the test became difficult to satisfy, and the courts adopted the position that the scope of the plea was not to be further widened (e.g. Carraher v HM Advocate (1946) JC 108 held that the plea was not available to a person suffering from psychopathic personality. But in Galbraith v HM Advocate (2002) JC 1 it was held that the formula in Savage was not to be read in a narrow sense, and it was not necessary that all the criteria in that formula had to be present. Furthermore, although the plea had to be
based on some form of mental abnormality, that condition need not be one bordering on insanity. Instead the court ruled that diminished responsibility required the existence of an abnormality of mind which had the effect that the accused's ability to determine or control his actings was substantially impaired. However, the Court excluded from the scope of the plea:
- any condition brought on by the consumption of drink or drugs, and
- psychopathic personality disorder.

# Australia and India
At present, Diminished Responsibility exists as a statutory partial defence in most Australian jurisdictions. The defence is only available in cases of murder and serves to reduce the offence to manslaughter.
For a comprehensive summary, see Law Commission. Partial Defences to Murder: Overseas Studies Consultation Paper No 173 (Appendices) 

# Notes
-  Jump up to: 1.0 1.1 Pogash, Carol (November 23, 2003). "Myth of the 'Twinkie defense': The verdict in the Dan White case wasn't based on his ingestion of junk food". San Francisco Chronicle. Retrieved 2007-08-10..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"\"""\"""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{display:none;font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
-  Snopes: The Twinkie Defense