Personality disorders in offenders with intellectual disability: a comparison of clinical, forensic and outcome variables and implications for service provision

Aim To establish any differences between patients with and without a diagnosis of personality disorders, being treated in a secure inpatient service for offenders with intellectual disability (ID) in the UK. Method A cohort study involving a selected population of people with ID and offending behaviours. Results The study included a total of 138 patients, treated over a 6 year period – 77 with a dissocial or emotionally unstable personality disorder and 61 without. Women were more likely to be in the personality disorder group. Both groups had high prevalence of abuse with no significant differences. Depressive disorders and substance abuse were more common in the personality disorder group, while epilepsy and autistic spectrum disorders were more common in the non-personality disorder group. Rather than differences, what was more striking was the rate and range of these comorbidities across both groups. Although past histories of violence and institutional aggression were no different, compulsory detention under criminal sections and restriction orders were more common in the personality disorder group. There were no differences in treatment outcomes. Conclusions Although about half of patients detained in secure units for offenders with ID have a personality disorder, there were more similarities than differences between this group and the rest. While good treatment outcomes supported the case for specialised secure treatment units for people with ID, the case for establishing a more specialised ID–personality disorder unit was less convincing. There is also a need to explore whether there are alternative diagnostic models that can delineate better the group with personality difficulties in this population.