Diminished Responsibility

Section 2(1) of the Homicide Act 1957, as amended, provides that a person 'who kills or is a party to the killing of another is not to be convicted of murder if D was suffering from an abnormality of mental functioning which — (a) arose from a recognised medical condition, (b) substantially impaired D's ability to do one or more of the things mentioned in subsection (1A), and' provides an explanation for the killing. Subsection (1A) specifies that those things are 'to understand the nature of D's conduct', 'to form a rational judgment', and 'to exercise self-control'. Subsection (1B) further provides that an abnormality of mental functioning provides an explanation for the conduct 'if it causes, or is a significant contributory factor in causing,' the defendant to carry out the killing.

The burden of proving the defence lies with the defendant, not the prosecution: section 2(2) states that 'On a charge of murder, it shall be for the defence to prove that the person charged is by virtue of this section not liable to be convicted of murder.' Where the defence succeeds, section 2(3) provides that the person 'shall be liable instead to be convicted of manslaughter' rather than murder. CPS legal guidance confirms that a defendant relying on the defence must establish that they were 'suffering from an abnormality of mental functioning', that it arose from a 'recognised medical condition', that it 'substantially impaired the defendant's ability either to understand the nature of their conduct or to form a rational judgment or to exercise self-control (or any combination)', and that it 'provided an explanation for their conduct'.