Diminished responsibility


Diminished responsible was introduced by the Homicide Act 1957 and amended by the Coroners and Justice Act 2009.

What is diminished responsibility?

It is a partial defence to murder.

A person who kills or is a party to the killing of another is not to be convicted of murder if the defendant was suffering from an abnormality of mental functioning which:

  • Arose from a 'recognised medical condition'
  • Substantially impaired the defendant's ability to do one of the following;  to understand the nature of D's conduct; to form a rational judgement or to exercise self-control. 
  • It provides an explanation for the D's acts and omissions in doing the killing or being a party to the killing. The abnormality of mental functioning must be a significant contributory factor in causing the defendant to carry out the conduct. 

Abnormality of mental functioning:

The definition of abnormality of mental functioning comes from the case Byrne(1960)...

'A state of mind so different from that of ordinary human beings that the reasonable man would term it abnormal' 

Arose from a recognised medical condition:

A recognised medical condition covers both psychological conditions, for example depressive illness, paranoia or battered women's syndrome and physical conditions which affect mental functioning such as epilepsy, sleep disorders or diabetes. 

There must be medical evidence given at the trial of an abnormality of mental functioning arising from a…


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