Diminished responsibility

Section 2 of the Homicide Act 1957 states: (1) Where a person kills or is party to a killing of another, he shall not be convicted of murder if he was suffering from an abnormality of mental functioning which -

(b) substantially impaired D's ability to do one or more of the things mentioned in subsection (1A), and

(c) provides an explanation for D's acts and omissions in doing or being a party to the killing.

(1B) For the purposes of subsection (1)(c), an abnormality of mental functioning provides and explanation of D's conduct if it causes, or is a significant contributory factor in causing, D to carry out that conduct.

It is very difficult to put it in a phrase, but it has been put in this way: that there must be aberration or weakness of mind; that there must be some form of mental unsoundness; that there must be a state of mind which is bordering on, though not amounting to, insanity; that there must be a mind so affected that responsibility is diminished from full responsibility to partial responsibility. In other words, the prisoner in question must be only partially accountable for his actions. And I think one can see running through the cases that there is implied ... that there must be some form of mental disease.