The legal intersection of psychosis and substance use: A mixed-methods investigation of settled insanity

Substance use and psychosis frequently co-occur and are complexly interrelated. Nevertheless, the legal system sometimes requires conclusions regarding the causal effects of substance use on a defendant’s psychosis (e.g., to determine insanity defense eligibility - including “settled” insanity from chronic substance use). Objectives: We sought to understand how courts and mental health professionals conceptualize and resolve settled insanity claims. Hypotheses: Mental health professionals’ approach to settled insanity cases will map well onto the relevant body of clinical psychological science. Method: We conducted a case law review (Study 1: N=51 settled insanity cases), coding history and outcomes, substances involved, viability from a clinical science perspective, and qualitative reasons for case outcomes. And we conducted an experiment to evaluate the perspectives of mental health experts on the legal intersection of substance use and mental illness across three types of cases (Study 2: N=310 U.S. licensed psychologists, 52% female, M_age=37.66, M_experience=18.71 years). Results: In Study 1, 29.41% of the settled insanity cases were potentially viable on clinical science grounds, whereas only a single legal case (1.96%) was actually successful. Qualitative analyses showed courts often use circular reasoning to disallow settled insanity based on the very characteristics that define it. Study 2 found mental health experts evaluate evidence of permanent mental illness similarly, regardless of whether a legal defense of insanity is general or settled in nature. And they largely opine defendants who are actively and voluntarily intoxicated at the time of the crime are not legally insane, consistent with the law. Conclusions: The legal doctrine of settled insanity does not map well onto clinical reality and unnecessarily complicates the intersection of intoxication and mental illness by prompting insoluble questions about the etiology of mental illness.