Evaluating Predominant Causes of Insanity in Cases of Drug-Induced Psychoses

The insanity defense operates on the basis that individuals who do not have the capacity to understand the consequences or wrongness of their action should not be held criminally responsible because there is a defect of reason. The defect must arise from a disease of mind' having internal causes. This raises questions about the way the law deals with externally caused defects of reasonsuch as drug-induced psychosesbecause it depends on whether voluntary intoxication caused acute psychosis and ensuing insanity directly linked to voluntary intoxication; triggered or exacerbated an existing psychotic mental illness; or triggered a psychotic illness that was fixed and permanent due to chronic substance abuse but is independent of acute intoxication: a case of settled insanity.' The latter two conditions can form a sufficient basis for the defense. But can cognitive impairments arising from mental disorders be distinguished from those arising from substance abuse disorders? How are such distinctions made? This article uses an Australian case to illuminate the nuanced operations of the defense because a verdict of not guilty because of mental impairment' was reached even though the defendant had no prior history of mental illness and had consumed cocaine and methamphetamine prior to his killings. Evaluators should consider the predominant rather than precise cause of insanity in cases in which voluntary intoxication and mental illness are pertinent issues for the purposes of the defense.