substantially impaired

• The abnormality of mental functioning must substantially impair the defendant's mental responsibility for their acts or omissions in being a party to or doing a killing.
• in r v Byrne, the appeal corut said that the question of whether the impairment was substantial was one of degree and it is a matter for the jury to decide
• In the case of r v Lloyd, it was held that substantial does not mean total, nor does it mean trivial or minimal. it is something in between for the jury to decide if the defendant's mental responsibility is impaired and if so, whether it is substantially impaired.
• Section 2 (1) (A) of the homicide act 1978 sets out that the defendant's ability to do one of the following things must be substantially impaired:

  • understand the nature of their conduct
  • form a rational judgment
  • exercise self control
• Ability to understand the nature of their conduct:

  • This covers situations such as where the defendant is in an automatic state, doesn't know what they are doing or is suffering from delusions. it also covers people with severe learning disabilities.
• Ability to form a rational judgement:

  • Those suffering from paranoia, schizophrenia or battered spouse syndrome may not be able to form a rational judgment and so the jury may consider all relevant circumstances before or after the killing.
• The ability to exercise self control:

  • This was the situation in the case of r v Byrne, Byrne was a sexual psychopath who wasn't able to control his perverted desires. The defence of diminished responsibility was therefore available to him.