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Why IQ is not a covariate in cognitive studies of neurodevelopmental disorders

  • MAUREEN DENNIS (a1) (a2), DAVID J. FRANCIS (a3) (a4), PAUL T. CIRINO (a3) (a4), RUSSELL SCHACHAR (a1) (a5), MARCIA A. BARNES (a6) and JACK M. FLETCHER (a3)...


IQ scores are volatile indices of global functional outcome, the final common path of an individual’s genes, biology, cognition, education, and experiences. In studying neurocognitive outcomes in children with neurodevelopmental disorders, it is commonly assumed that IQ can and should be partialed out of statistical relations or used as a covariate for specific measures of cognitive outcome. We propose that it is misguided and generally unjustified to attempt to control for IQ differences by matching procedures or, more commonly, by using IQ scores as covariates. We offer logical, statistical, and methodological arguments, with examples from three neurodevelopmental disorders (spina bifida meningomyelocele, learning disabilities, and attention deficit hyperactivity disorder) that: (1) a historical reification of general intelligence, g, as a causal construct that measures aptitude and potential rather than achievement and performance has fostered the idea that IQ has special status and that in studying neurocognitive function in neurodevelopmental disorders; (2) IQ does not meet the requirements for a covariate; and (3) using IQ as a matching variable or covariate has produced overcorrected, anomalous, and counterintuitive findings about neurocognitive function. (JINS, 2009, 15, 331–343.)


Corresponding author

*Correspondence and reprint requests to: Maureen Dennis, Program in Neurosciences and Mental Health, Department of Psychology, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8. E-mail:


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