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Mol Autism. 2019 Mar 1;10:9. doi: 10.1186/s13229-019-0260-x. eCollection 2019.

Selection bias on intellectual ability in autism research: a cross-sectional review and meta-analysis.

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1College House, University of Exeter Medical School, University of Exeter, Exeter, EX1 2LU UK.
2UCL Research Department of Clinical, Educational and Health Psychology, Gower Street, London, WC1E 6BT UK.
3College of Social Science and International Studies, Byrne House, University of Exeter, Exeter, EX4 4PJ UK.
4Brain in Hand, Innovations Centre, University of Exeter, Exeter, EX4 4QJ UK.



Current global estimates suggest the proportion of the population with autism spectrum disorder (ASD) who have intellectual disability (ID) is approximately 50%. Our objective was to ascertain the existence of selection bias due to under-inclusion of populations with ID across all fields of autism research. A sub-goal was to evaluate inconsistencies in reporting of findings.


This review covers all original research published in 2016 in autism-specific journals with an impact factor greater than 3. Across 301 included studies, 100,245 participants had ASD. A random effects meta-analysis was used to estimate the proportion of participants without ID. Selection bias was defined as where more than 75% of participants did not have ID.


Meta-analysis estimated 94% of all participants identified as being on the autism spectrum in the studies reviewed did not have ID (95% CI 0.91-0.97). Eight out of ten studies demonstrated selection bias against participants with ID. The reporting of participant characteristics was generally poor: information about participants' intellectual ability was absent in 38% of studies (n = 114). Where there was selection bias on ID, only 31% of studies mentioned lack of generalisability as a limitation.


We found selection bias against ID throughout all fields of autism research. We recommend transparent reporting about ID and strategies for inclusion for this much marginalised group.


Autism; Autism spectrum disorder; Intellectual disability; Nosology; Selection bias

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