Journal of the International Neuropsychological Society

Critical Review

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

MAUREEN DENNISa1a2 c1, DAVID J. FRANCISa3a4, PAUL T. CIRINOa3a4, RUSSELL SCHACHARa1a5, MARCIA A. BARNESa6 and JACK M. FLETCHERa3

a1 Program in Neurosciences and Mental Health, Department of Psychology, The Hospital for Sick Children, Toronto, Ontario, Canada

a2 Department of Surgery, The Hospital for Sick Children, Toronto, Ontario, Canada

a3 Department of Psychology, University of Houston, Houston, Texas

a4 Texas Institute for Measurement, Evaluation, and Statistics, University of Houston, Houston, Texas

a5 Department of Psychiatry, University of Toronto, Toronto, Ontario, Canada

a6 Children’s Learning Institute, Department of Pediatrics, University of Texas Houston, Houston, Texas

Abstract

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.)

(Received September 15 2008)

(Reviewed December 29 2008)

(Accepted December 29 2008)

Correspondence:

c1 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: maureen.dennis@sickkids.ca