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Murder - defences - diminished responsibility

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Defence only available on charges of murder not attempted murder

The law on diminished responsibility is contained in section 2 of the Homicide Act 1957.

"Where a person kills or is a party to the killing of another, he shall not be convicted of murder if he was suffering from such

Introduced because

Problems with the very narrow definition of insanity under the M'Naghten rules.


Before the 1957 Act, over 40% of murder trials involved a plea of insanity. Now most defendants will opt of the defence of diminished responsibility.


DR is easier to prove, than insanity sometimes relying on little evidence, for example in Price (1971) D killed his severely handicapped son.


The following examples would not equate with insanity and before 1957 so a defendant would be found guilty of murder, which was thought to be harsh.


Diminished Responsibility had long been known to the Scottish Courts.


It is not available for attempted murder.


Examples of conditions held to amount to diminished responsibility

Reduces the offence to that of manslaughter

Allowing the judge to exercise discretion in sentencing.


About 30% receive prison sentences.  An alternative sentence is a hospital order under s.37 Mental Health Act 1983.


Jury to decide in new or borderline cases.

Since 1962 pleas of diminished responsibility are decided by the prosecution and judge, if they don‘t agree it goes to the jury. New or borderline case are left to the jury.


Peter Sutcliffe (The Yorkshire Ripper) pleaded diminished responsibility.  He murdered 13 women. Medical reports were unanimous in suggesting that he was a paranoid schizophrenic and the prosecution were prepared to accept diminished responsibility.

The judge decided that it was in the public interest for the jury to decide on the matter. The jury returned 13 verdicts of murder.

The jury's response reflects public reaction rather than any clear cut rule of law and it illustrates rather well that in practice medical evidence alone may not be sufficient for the defence to succeed. It appears they did not accept there was “substantially impaired responsibility‘ which is a moral as well as a medical decision.


Today only about 1 in 5 cases of diminished responsibility go to a jury.


Balance of probabilities

DR is raised by the defence, who have to prove it on the preponderance of probabilities, Dunbar [1958]. It is only argued in court when the prosecution rejects the plea or has counter evidence, for example that D is in fact insane.

The burden being on the defence does not breach the ECHR, Article 6 R v Lambert [2001] HL (a drug and murder cases conjoined).


The separate requirements

Abnormality of mind

In Byrne [1960] Lord Parker defined 'abnormality of mind' as:

"a state of mind so different from that of ordinary human beings that the reasonable man would term it abnormal. It appears to us to be wide enough to cover the mind's activities in all its aspects, not only the perception of physical acts and matters, and the ability to form a rational judgment as to whether an act is right or wrong, but also the ability to exercise willpower to control physical acts in accordance with that rational judgment."

Byrne was a sexual psychopath who found it difficult, if not impossible, to control his perverted sexual desires he strangled a young woman and horrifyingly mutilated the body.


So, irresistible impulses are capable of amounting to diminished responsibility.


"Abnormality of mind", which has to be contrasted with the expression "defect of reason"

It appears to cover the mind’s activities in all its aspects.
It is not limited to the perception of physical acts and matters, and the ability to form a rational judgment whether an act is right or wrong. 

It includes the ability to exercise will-power to control physical acts in accordance with rational judgment R v Byrne [1960] CA.


It is not necessary to show that the accused’s abnormality existed from birth R v Gomez [1964] CA.


Abnormality of mind induced by alcohol or drugs is not due to inherent causes

R v Gittens [1984] CA & R v Fenton (1975) CA.

If drinking alcohol has reached the stage that the accused’s brain has been damaged so that there is gross impairment of judgment, or the accused drinks alcohol because he can no longer resist the impulse to drink, the defence of diminished responsibility is available R v Tandy [1989] CA.


If D was intoxicated during a killing and also has an abnormality of the mind, the intoxication can contribute to his diminished responsibility; R v Dietschmann [2003]. He does not have to show he would still have killed if sober.


A condition of arrested or retarded development of mind or any inherent causes or induced by disease or injury

This includes absence not only of the ability to tell right from wrong but also of the willpower to control physical acts.

Sec 2 restricts the defence so it is not available to people who kill because of emotions such as hate or jealousy (but note that so called "Othello Syndrome" is a more serious form of jealousy and was allowed, but this is doubted; R v Vinagre 1979 CA).


Depression or a medical disorder can be linked to either an “inherent cause‘ or “disease‘

Diminished responsibility must occur because of one of the reasons in the list any other cause will not suffice. The Act does not provide that a defence of diminished responsibility must be based on medical evidence, such a defence is not likely to succeed without such evidence R v Dix (1981) CA. 

Lord Parker in R v Byrne [1960]

"medical evidence is no doubt of importance, but the jury are entitled to take into consideration all the evidence, including the acts or statements of the accused and his demeanour. They are not bound to accept the medical evidence if there is other material before them which, in their good judgment, conflicts with it and outweighs it" .

‘arrested or retarded development’
Is assumed to mean what was formerly referred to as ‘mentally subnormal’ individuals R v Egan [1992].

‘Inherent cause’ is widely assumed to allude to functional mental illness as opposed to organic disease or injury R v Sanderson (1993),


The issue which has arisen in connection with the defence is what is meant by disease in this context. Is battered woman’s syndrome a disease or injury?


There is a medical list – the British Classification of Mental Diseases – of recognised mental diseases and currently it does include battered women’s syndrome R v Hobson [1998].


Alcoholism at the level of an irresistible impulse to take drink can give rise to a defence of diminished responsibility R v Tandy (1987). However, mere intoxication on a particular occasion or desire for alcohol or drugs that falls short of ‘irresistible’ cannot be regarded as a disease or an injury for these purposes R v O’Connell [1997].


If someone is intoxicated and also suffering from arrested or retarded development (for example) the jury does not have to consider what the effect would have been if the defendant were not intoxicated R v Dietschmann [2003].


Injury is usually taken to apply to organic or physical condition of the brain or parts of the body which can affect behaviour.


Substantially impaired

The abnormality of the mind had to be such as to substantially impair the defendant's responsibility for his actions.

But this does not require that the defendant is not at all responsible for his actions.


‘Substantially’ allows the jury to consider the issue of degree.


The word means more than some trivial degree of impairment but less than total impairment R v Lloyd [1967].


‘Substantially’ suggests some quantitative assessment, which suggests a moral judgment.  Medical witnesses feel uneasy about testifying about responsibility which is a  legal or moral concept but do so in order that the defence can work.


Impaired mental responsibility

‘Mental responsibility for his acts’

Means the extent to which the accused’s mind is answerable for his physical acts which must include a consideration of the extent of his ability to exercise will-power to control his physical acts R v Byrne [1960] CA.

Diminished responsibility is not a medical diagnosis, it is a legal concept which ultimately only a jury can decide

The state of mind must by a condition of arrested development of the mind or any inherent causes along with disease or injury.


Medical evidence from at least two psychologists is also required.  Medical evidence is important but the jury is not bound to accept it.

Whether the abnormality of mind was sufficiently substantial to impair his mental responsibility is a question of degree.


Abnormalities from taking drugs or alcohol alone cannot form the basis for a defence of diminished responsibility.


An abnormality which ‘substantially impairs his mental responsibility’ involves a mental state which in popular language a jury would regard as amounting to partial insanity or being on the borderline of insanity R v Byrne.

In R v Rose [1961] PC, a warning was given about directing a jury that ‘borderline of insanity’ could be equated with ‘abnormality of mind’ (case of chronic reactive depression)).


The abnormality of mind has to substantially impair the defendant’s mental responsibility for his acts or omissions.

By contrast, insanity is concerned with the ability to appreciate the nature and quality of the act.


Diminished Responsibility includes:

  • the perception of physical acts and matters

  • the ability to form a rational judgment whether an act is right or wrong,

  • the ability to exercise will-power to control physical acts in accordance with that rational judgment.

The jury can take into account the medical evidence, the conduct of the accused at the time of the killing, and his statements, acts and demeanour. They are not bound to accept the medical evidence.


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