The Insanity Defence

Abstract Insanity is a defence with a lengthy history in the common law of England. But it has also been the subject of much debate, and ultimately law reform, in many other jurisdictions. Typically, the defence, as exemplified in the M’Naghten Rules, represents a foundational aspect of criminal responsibility, although in some jurisdictions it serves only to define degrees of mental capacity. But what all jurisdictions have in common is the high and increasing incidence of mental illness and impairment challenging existing constructions of an exculpatory rule. Acknowledging this fact, this book brings together a number of experts from a range of jurisdictions to provide fresh perspectives on the insanity defence. While the origins and operation of the M’Naghten Rules are explored in detail in several chapters, other chapters highlight the eclectic nature of the defence, its highly variable linguistic expression, and the diverse social policy mandates it seeks to embrace. Such policies may move, in different jurisdictions, between excluding criminal liability because of lack of capacity and reduced culpability because of impaired capacity so that insanity becomes primarily a sentencing issue. In other jurisdictions, while substantive insanity rules may bear many similarities to other jurisdictions, more demanding evidential and procedural requirements may significantly limit the scope of the exculpatory rule. The highly contested question of the appropriate disposition of offenders found legally insane arises both in international and domestic law and is also addressed in several chapters in the book. In some jurisdictional settings, notably in the context of international criminal law, a mixing of the separate constructs of trial capacity (unfitness to stand trial) and criminal incapacity (insanity) has had the potential to cause confusion, especially where both issues are raised simultaneously at a trial. Unfitness to stand trial is not discussed here but was the subject of investigation in an earlier volume entitled Fitness to Plead – International and Comparative Perspectives, Oxford, 2018. It is hoped this book will become a benchmark for theory and practice around the insanity defence while providing a coherent and detailed account of its diverse influence and manifestations across a wide range of jurisdictions.