Law and Human Behavior, Vol 49(6), Dec 2025, 532-555; doi:10.1037/lhb0000625
Objective: 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 psychosis (e.g., to determine insanity defense eligibility—including “settled” insanity from chronic substance use). We sought to understand how courts and mental health professionals conceptualize and resolve settled insanity claims. Hypotheses: Study 1 had no a priori hypotheses. Study 2 hypothesized a successful insanity defense would be seen as most possible for defendant when no substance use was implicated and least possible for defendants acutely intoxicated at the time of crime, with settled insanity falling between. 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, Mage = 37.66, Mexperience = 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 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 complicates the intersection of intoxication and mental illness by prompting insoluble questions about the etiology of mental illness. (PsycInfo Database Record (c) 2025 APA, all rights reserved)