Prevalence of criminal offending by men and women with intellectual disability and the characteristics of offenders: implications for research and service development.

The investigation of the relationship between criminal offending and the presence of an intellectual disability (ID) is problematic for two main reasons. First, because of problems associated with the definition of 'ID' and secondly, because much criminal offending goes undetected or unreported, and studies can only investigate those already involved with the criminal justice process. Studies using IQ as a continuous variable indicate that significantly below-average intellectual ability is an independent predictor of future offending. Whilst people with ID may be over-represented in parts of the criminal justice system, given the intellectual and other psychosocial disadvantages which they experience, the level of offending behaviour in this particularly vulnerable group is strikingly low. The present authors propose that two broad groups of people can be identified. The first, broader, group is one of people for whom social disadvantage and mental ill health (particularly substance abuse), coupled with a significant intellectual impairment, are the main characteristics. Secondly, there is a smaller group of people, usually already known to ID services as service users, but for whom the process whereby what might have been conceptualized as 'challenging behaviour' becomes 'offending' is far from clear. The distinction the present authors make between challenging behaviour and offending is important for understanding how 'difficult' behaviour becomes identified as 'antisocial/criminal behaviour'. They argue that research needs to move from prevalence and descriptive studies to investigating the processes which determine movement in and out the criminal justice system. The present political emphasis on public protection and proposals for significantly broader mental health legislation raise the danger of a re-expansion of institutional models of care, rather than the development of multi-agency support networks. The present paper underscores a note of caution, particularly where choices have to be made between expanding institutional models on the one hand and providing more integrated services on the other. Over and above policy decisions, these are social and political choices.