Two studies on the prevalence and validity of personality disorder in three forensic intellectual disability samples

Abstract

There is an extensive research literature on the association between personality disorder, antisocial personality disorder, and risk of future violent and sexual offences. Several studies have found an elevated prevalence of personality disorder diagnoses amongst those individuals with severe mental illness and criminal populations. While there has been some work on the prevalence of personality disorder among intellectual disability populations, it has been criticised as being unreliable and inconsistent. The present authors have taken account of these criticisms and recommendations in this comparison of 164 offenders with intellectual disability across three settings - community, medium/ low secure, and high secure. In Study 1, DSM-IV diagnoses were made on the basis of four information sources: file review, interview with clinician, observations by care staff, and the Structured Assessment of Personality Interview. Across the samples, total prevalence of PD was 39.3%. The most common diagnosis was antisocial personality disorder. There was a higher rate of diagnosis in the high security setting, with no significant differences between the other two settings. There was no diagnosis of dependent PD, indicating that assessors were not overly influenced by the developmental disability itself. In Study 2 it was found that increase in severity of PD (as indicated by PCL-R scores and/ or the number of PD diagnoses) showed a strong lawful relationship with instruments predicting future violence (VRAG, RM 2000/V) and a weaker relationship with instruments predicting future sexual offences (Static-99, RM 2000/S). The results indicate the utility of PD classification in this client group and that a number of individuals with PD classification are being managed successfully in community settings. These findings have considerable implications for staffing, both in terms of which individuals can be treated by these services and staff training.