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

Ginny Russell, William Mandy, Daisy Elliott, Rhianna White, Tom Pittwood, Tamsin Ford

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175 Citations (SciVal)


Background: 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. Methods: 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. Results: 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. Conclusions: 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.

Original languageEnglish
Article number9
JournalMolecular Autism
Issue number1
Publication statusPublished - 1 Mar 2019

Bibliographical note

Funding Information:
We would like to thank to Darren Moore, Hateem Rafeeque, Delphine Jacobs and Ruth Gwernan-Jones for their help and advice on the study, and the generous support of the Wellcome Trust, Grant ref.: 108676/Z/1 SfZ.

Funding Information:
This work was supported by a Wellcome Trust Investigator Award, Grant ref.: 108676/Z/1 SfZ. The funders had no role in development of the work.

Publisher Copyright:
© 2019 The Author(s).

Copyright 2019 Elsevier B.V., All rights reserved.


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

ASJC Scopus subject areas

  • Molecular Biology
  • Developmental Neuroscience
  • Developmental Biology
  • Psychiatry and Mental health


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