Patterns of offending among people with intellectual disability: a systematic review. Part I: methodology and prevalence data.

A systematic review of research on offenders with intellectual disability (ID) was conducted. In the present study, the first of a two-part presentation of the findings, the authors outline the methodology of the review and present data on the prevalence of offending by adults with ID. The review highlights the methodological problems of the research and the low level of rigour in many of the studies. The organization of the penal and 'care' systems are seen to have a huge impact on research findings. In addition, studies which adopt an IQ-based concept of ID show low rates of offending, whilst those which use wider definitions (e.g. attendance at special school) show higher ones. There is also preliminary evidence for believing that the prevalence of arson and sexual offences may be higher relative to other kinds of crimes for people with ID than for other offenders.