Introduction

In this chapter, we consider the general defence of insanity which is available in relation to all crimes, and the additional partial defence of diminished responsibility, which is available only to reduce a murder charge to manslaughter. It is widely accepted that the law on insanity is inadequate (see eg the Committee on Mentally Abnormal Offenders, 1975, 217–19). The general defence is too narrow in its test for insanity and too broad in terms of its concept of disease. The result of this is that many defendants whom most people would regard as insane cannot use the defence, while some whom most would regard as perfectly sane are forced to use it if they want to deny responsibility for a crime. For the former group, the additional partial defence of diminished responsibility operates, but only to scale murder down to manslaughter. For the latter group, it is sometimes better to plead guilty to the charge than to have one’s physical condition (for example, epilepsy: Sullivan (1983); or diabetes: Hennessy (1989)) forced into the law’s pigeonhole of insanity.