Moral responsibility and mental disorder: A philosophical exploration of the insanity defence

People are assumed to have moral responsibility, but the presence of mental disorder may call this into question. Nevertheless, when and how mental disorder affects moral responsibility is unclear. The insanity defence exculpates a defendant and effectively extinguishes their moral responsibility, although the criteria for insanity varies between different jurisdictions. Most successful cases of insanity are due to delusions, although the nature of delusions itself varies considerably. The recent case of Keal in England and Wales demonstrates some of the problems with the insanity defence, and a coherent philosophical basis for the defence is lacking. I argue that a volitional element should be added to the insanity defence in England and Wales such that a person may be exculpated on the basis of a mental disorder which sufficiently causes any of (1) defects of reason; (2) deficits in knowledge; (3) a lack of control; and (4) their ability to form intentions in crimes of intent, although the threshold should remain high. Reasons-responsive accounts of moral responsibility underpin many of these abilities. The extent to which mental disorder, particularly delusions, are considered a type of manipulation, rather than part of the self, is likely to be contributing to intuitions about exculpation although not all delusions will cause impairments or manipulation to the same extent. Amending the insanity defence in this way would be more just, more philosophically coherent and allow a more consistent approach for those with mental disorders who are charged with criminal offences.