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The defence of diminished responsibility is a special defence (only available to a charge of murder) and partial defence (if successful, it reduces the conviction to manslaughter). This defence is widely used because murder has a mandatory life sentence. To succeed, the defendant must show that, on the balance of probabilities, the elements of diminished responsibility are in place. These are defined in s.2 of the Homicide Act 1957:
- at the time of the killing the defendant was suffering from an "abnormality of mind" arising from "arrested or retarded development" or "induced by disease or injury"
- the abnormality led to the fatal act, and
- the abnormality resulted in a "substantial" impairment of the defendant's mental responsibility for his actions (R v Lloyd  CA).
In practice it is necessary to get medical evidence in support of such a claim. Even then the prosecution may seek a ruling of insanity rather than dimished responsibility because, even though a defendant who is found not guilty by reason of insanity is not convicted of any offence, he can be ordered to be detained for psychiatric treatment. A successful plea of diminished responsibility could result in a short prison sentence, followed by unsupervised release.
That diminished responsibility exists at all as a defence is probably because murder carried a mandatory death penalty until 1963.
R, R. v 4 (02 February 2010)
R, R. v  EWCA Crim 194 (02 February 2010) The single issue was whether the appellant's responsibility for his actions in killing the victim was substantially impaired so as to enable the jury to return a verdict of manslaughter on the grounds of diminished responsibility.
In R v Lloyd  1 QB 175 the Court of Criminal Appeal considered directions given in two separate cases: R v Simcox (The Times, 25 February 1964) and in R v Lloyd. In Simcox Finnemore J directed the jury to look at the issue
- "in a broad common-sense way and ask yourselves, having heard what the doctors have said, .... knowing the whole story, 'Do we think, looking at it broadly as common-sense people, there was a substantial impairment of his mental responsibility in what he did?' .... If the answer is 'no', there may be some impairment, but we do not think it was substantial, we do not think it was something that really made any great difference, although it may have made it harder to control himself, to refrain from crime, then you would find him guilty ...."
That direction was approved in the Court of Criminal Appeal, presided over by Lord Parker CJ. In Lloyd Ashworth J directed the jury that their common sense would tell them what "substantial impairment" meant. He went on:
- "I am not going to try to find a parallel for the word 'substantial'. You are the judge, but your own common sense will tell you what it means. This far I will go. Substantial does not mean total, that is to say, the mental responsibility need not be totally impaired, so to speak, destroyed altogether. At the other end of the scale substantial does not mean trivial or minimal. It is something in between and Parliament has left it to you and other juries to say on the evidence, was the mental responsibility impaired, and, if so, was it substantially impaired?"
This direction was criticised in the Court of Criminal Appeal on the basis that it was misleading because "substantially" can mean either that the impairment was "real and not illusory" or that it was of a "considerable amount". Accordingly, the jury should have been directed that the word "substantially" meant "real or really impaired". However, faced with that contention, the court pointed out to leading counsel for the appellant that the word "substantially" need not appear at all if he was correct. So counsel's submission was modified to "substantially, meaning something more than trivial".
Pausing there for a moment, there is a similar series of contentions in this case. When the two directions given by Judge Devaux are analysed, first in his summing-up and then in answer to the questions raised by the jury after their retirement, it will be seen that there is very little difference between the issues canvassed in Lloyd and the way in which Judge Devaux dealt with the matter.
The Court of Appeal rejected the submission in Lloyd. Like the court in Simcox, which could see no ground for criticising the judge's direction in that case, there were no grounds for criticising the judge in this. It was accepted that the words used by the different judges in the two directions they had given to the jury were not identical, but the substance of the directions in both cases seemed to be "for all substantial purposes indistinguishable one from the other".
That presented Mr Glen's argument with a significant problem. However, he drew our attention to some observations in R v Egan (1992) 95 Cr App R 278. That case is better known for its analysis of the impact of drunkenness on the defence of diminished responsibility, which has now been overruled on that point. However, at the very end of the judgment the court referred to the issue of "substantial impairment" and expressly advised judges that
".... guidance as to the meaning of 'substantial' should be explicitly provided for the jury by using one or other of the two meanings in Lloyd."