The insanity defense represents one of the most controversial and debated evaluations performed by forensic psychiatrists and psychologists. Despite the variation among different jurisdictions, in Western countries, the legal standards for insanity often rely on the presence of cognitive and/or volitional impairment of the defendant at the time of the crime. We developed the defendant's insanity assessment support scale (DIASS) based on a wide view of competent decision-making, which reflects core issues relevant to legal insanity in many jurisdictions. To assess the characteristics of the DIASS we asked 40 forensic experts (16% women; years of experience = 20.6 ± 12.9) to evaluate 10 real-life derived forensic cases with the DIASS; cases included defendants' psychiatric symptom severity, evaluated through the 24-itemBrief Psychiatric Rating Scale (BPRS). Exploratory factor analysis by principal axis factoring was conducted, which disclosed a two-factor solution explaining 57.6% of the total variance. The DIASS showed a good internal consistency (Cronbach's alpha = 0.86), and substantial inter-rater reliability (Cohen's kappa = 0.72). The capacities analyzed through the DIASS were mainly affected by mania/excitement and psychotic dimensions in nonresponsible and with substantially diminished responsibility defendants, while by hostility and negative symptoms in responsible defendants. The DIASS proved to be an effective psychometric tool to guide and structure insanity defense evaluations, in order to improve their consistency and reliability.
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