Diminished responsibility and the criminal law

Diminished responsibility is one of three special defences which exist for the criminal offence of murder. The defence of diminished responsibility is set out in s 2 of the Homicide Act 1957 (HA 1957) (as amended by the Coroners and Justice Act 2009 (CJA 2009)). When the defence is successfully pleaded it has the effect of reducing a murder conviction to manslaughter.

Diminished responsibility is part of three special defences which differ from all other defences in the criminal law as they only apply to murder and have the effect of reducing criminal liability, rather than absolving the defendant from liability completely. Also contained within this three-pronged group are the defences of provocation and suicide pact.

It is for the defence to prove that the person is not liable to be convicted of murder. The evidential burden is on the defence on the balance of probabilities.

Section 2 of HA 1957, as amended by CJA 2009, sets out the requirements which must be established by the defendant to succeed with the defence of diminished responsibility. The defendant must be suffering from an abnormality of mental functioning which:

• arose from a recognised medical condition;
• substantially impaired the defendant’s ability to do one or more of the following:
  • understand the nature of their conduct;
  • form a rational judgment;
  • to exercise self-control.
• provides an explanation for defendant’s acts and omissions in doing or being a party to the killing.

Under HA 1957, the defendant must have been suffering from an abnormality of the mind; whereas under CJA 2009, the defendant must be suffering from an abnormality of mental functioning. The change of wording does not substantially alter the applicability of the defence, so the precedents set under the previous law are still valid.

To establish whether a defendant is suffering from an abnormality of mental functioning, medical evidence will be provided in court; it will, however, be up to the jury to decide whether the defendant is suffering from an abnormality of mental functioning and whether the defence of diminished responsibility should succeed; they are not bound to accept the expert evidence (R v Sanders (1991)). However, if there is unchallenged medical evidence of diminished responsibility and no other evidence is able to rebutt the defence, the trial judge should withdraw a charge of murder from the jury (R v Brennan (2014)).

The test which will be required to establish an abnormality of mental functioning is that by which a reasonable person would regard as abnormal. This test has a very wide meaning and includes the ability to exercise will power and control.

The following have been held to be an abnormality of mental functioning in cases of diminished responsibility:

• jealousy (R v Miller (1972));
• pre-menstrual tension (R v Reynolds (1988));
• battered woman syndrome (R v Ahluwalia (1993));
• epilepsy (R v Campbell (1997));
• chronic depression (R v Gittens (1984)).

To establish an abnormality of mental functioning and to use the defence of diminished responsibility it is not sufficient to simply have the condition. The defendant must prove that the condition was excessive when compared to that experienced by a reasonable person.

Under s 2(1)(a) of HA 1957 (as amended by CJA 2009), the defendant’s abnormality of the mind must have been brought on by a recognised medical condition.

The abnormality must have been caused by a source which comes from inside that person and not a factor which is said to be outside them. Outside factors which cause the abnormality, such as binge drinking or occasional drug-taking, would not therefore be enough to establish diminished responsibility (R v Fenton (1975)).

However, drug addiction and alcoholism are seen as medical diseases so these would be taken into account if the abnormality was brought about as a result of the disease of alcoholism or drug addiction or if it is due to the long term damage caused by the intake of such substances (R v Wood (2009)). The same approach will be taken if the defendant was intoxicated by prescription drugs (R v O’Connell (1997)).

Whether the abnormality of the mind substantially impaired the defendant’s ability to: understand the nature of their conduct; form a rational judgment; or exercise self control under HA 1957, s 2(1)(b), will be a question of fact for the jury.

It requires that the defendant was not fully aware of their actions due to their mental state. Any evidence of planning to commit the crime will demonstrate that the defendant’s mental responsibility was not impaired. Therefore, the defence of diminished responsibility will not apply to a premeditated attack, only a murder which happened on the spur of the moment.

The defendant will be able to satisfy the requirement under HA 1957, s 2(1)(c), that the abnormality of mental functioning provides an explanation for their conduct, if they can show the medical condition was the reason for them carrying out the act or that it was an important contributing factor. Where the defendant’s mental condition made no difference to their behaviour – ie, they would have killed regardless of their medical condition – the defence should not succeed.

Diminished responsibility is often a defence which will be stated on an appeal from a murder charge. However, the medical condition must usually have been cited in the original trial for it to be later relied on as a defence in the appeal hearing. The courts, however, will be more willing to allow fresh evidence relating to diminished responsibility where there have been advances in medical opinion since the time of trial (R v Ahluwalia (1993)).