Intellectual Disability amongst Individuals with Autism Spectrum Disorders: A Systematic Review and Meta-Analysis

Background: A key predictor of the type and level of support an individual with an Autism Spectrum Disorder (ASD) requires is the presence of an intellectual disability (ID). It is therefore important to establish the number... [ view full abstract ]

Background: A key predictor of the type and level of support an individual with an Autism Spectrum Disorder (ASD) requires is the presence of an intellectual disability (ID). It is therefore important to establish the number of individuals with ASD and co-occurring ID to plan for the support of the ASD population in the future. Previous estimates of this figure have varied significantly. The aim of this study was to generate a reliable, pooled estimate of the number of individuals with and without an ID to inform future planning.

Method: We carried out a systematic review of the literature to identify all peer reviewed, English language papers published between 2002 and 2013, which reported IQ scores of individuals with ASD or associated sub-diagnoses. 38 papers were identified and their quality determined by methodologies used to analyse their data. 33 studies of a low quality were excluded from further analysis, and a meta-analysis of the remaining 5 high quality studies was used to generate a pooled estimate of the distribution of IQ scores across the ASD population.

Results: The results of the meta-analysis indicated that 33% of individuals with ASD had an intellectual disability (95% C.I. 27-39%).

Conclusions: These results point to the need for a radical reappraisal of current estimates of ASD and intellectual disability. This study has also highlighted clinical and methodological issues which may account for variability in previously reported rates.