Hostname: page-component-848d4c4894-ndmmz Total loading time: 0 Render date: 2024-05-18T17:00:13.569Z Has data issue: false hasContentIssue false

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

Published online by Cambridge University Press:  01 May 2009

Program in Neurosciences and Mental Health, Department of Psychology, The Hospital for Sick Children, Toronto, Ontario, Canada Department of Surgery, The Hospital for Sick Children, Toronto, Ontario, Canada
Department of Psychology, University of Houston, Houston, Texas Texas Institute for Measurement, Evaluation, and Statistics, University of Houston, Houston, Texas
Department of Psychology, University of Houston, Houston, Texas Texas Institute for Measurement, Evaluation, and Statistics, University of Houston, Houston, Texas
Program in Neurosciences and Mental Health, Department of Psychology, The Hospital for Sick Children, Toronto, Ontario, Canada Department of Psychiatry, University of Toronto, Toronto, Ontario, Canada
Children’s Learning Institute, Department of Pediatrics, University of Texas Houston, Houston, Texas
Department of Psychology, University of Houston, Houston, Texas
*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:


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.)

Critical Review
Copyright © The International Neuropsychological Society 2009

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)



Aase, H., Meyer, A., & Sagvolden, T. (2006). Moment-to-moment dynamics of ADHD behaviour in South African children. Behavioral and Brain Functions, 2, 11. doi: 10.1186/1744-9081-2-11.CrossRefGoogle ScholarPubMed
Adams, K.M., Brown, G.G., & Grant, I. (1985). Analysis of covariance as a remedy for demographic mismatch of research subject groups: Some sobering simulations. Journal of Clinical and Experimental Neuropsychology, 7, 445462.CrossRefGoogle ScholarPubMed
Barkley, R.A., Murphy, K.R., & Bush, T. (2001). Time perception and reproduction in young adults with attention-deficit/hyperactivity disorder. Neuropsychology, 15, 351360.CrossRefGoogle ScholarPubMed
Barnes, M.A. & Dennis, M. (1992). Reading in children and adolescents after early onset hydrocephalus and in normally developing age peers: Phonological analysis, word recognition, word comprehension, and passage comprehension skill. Journal of Pediatric Psychology, 17, 445465.CrossRefGoogle ScholarPubMed
Barnes, M.A., Huber, J., Johnston, A.M., & Dennis, M. (2007). A model of comprehension in spina bifida meningomyelocele: Meaning activation, integration, and revision. Journal of the International Neuropsychological Society, 13, 854864.CrossRefGoogle Scholar
Binet, A. (1909/1975). Modern ideas about children; translated by Heisler, S.. (Original work published 1909). Menlo Park, CA: Suzanne Heisler.Google Scholar
Binet, A. & Simon, T. (1916). The development of intelligence in children (The Binet-Simon Scale); translated by Kite, E.S.. Training School. Vineland, NJ: Suzanne Heisler.CrossRefGoogle Scholar
Boonstra, A.M., Kooij, J.J.S., Buitelaar, J.K., Oosterlaan, J., Sergeant, J.A., Heister, J.G.A.M.A., & Franke, B. (2008). An exploratory study of the relationship between four candidate genes and neurocognitive performance in adult ADHD. American Journal of Medical Genetics Part B (Neuropsychiatric Genetics), 147B, 397402.CrossRefGoogle Scholar
Bowers, T.S. & Beck, B.D. (2006). What is the meaning of non-linear dose-response relationships between blood lead concentrations and IQ? Neurotoxicology, 27, 520524.CrossRefGoogle ScholarPubMed
Bridgett, D.J. & Walker, M.E. (2006). Intellectual functioning in adults with ADHD: A meta-analytic examination of full scale IQ differences between adults with and without ADHD. Psychological Assessment, 18, 114.CrossRefGoogle ScholarPubMed
Burt, C. (1937). The backward child. New York: Appleton-Century.Google Scholar
Campbell, D.T. & Erlebacher, A.E. (1970). How regression artifacts in quasi-experimental evaluations can mistakenly make compensatory education look harmful. In Hellmuth, J. (Ed.), Disadvantaged child: Vol. 3. Compensatory education: A national debate (pp. 185210). New York: Brunner/Mazel.Google Scholar
Campbell, D.T. & Kenny, D.A. (1999). A primer on regression artifacts. New York: Guilford Press.Google Scholar
Carroll, J.B. (1993). Human cognitive abilities: A survey of factor-analytic studies. New York: Cambridge University Press.CrossRefGoogle Scholar
Cattell, R.B. (1943). The measurement of adult intelligence. Psychological Bulletin, 40, 153193.CrossRefGoogle Scholar
Ceci, S.J. (1991). How much does schooling influence general intelligence and its cognitive components? A reassessment of the evidence. Developmental Psychology, 27, 703722.CrossRefGoogle Scholar
Craik, F.I.M. & Bialystok, E. (Eds.) (2006). Lifespan cognition: Mechanisms of change. New York: Oxford University Press.Google ScholarPubMed
Deary, I.J., Lawn, M., & Bartholomew, D.J. (2008). A conversation between Charles Spearman, Godfrey Thomson, and Edward L. Thorndike: The International Examinations Enquiry Meetings 1931-1938. History of Psychology, 11, 122142.CrossRefGoogle Scholar
Deese, J. (1993). Human abilities versus intelligence. Intelligence, 17, 107116.CrossRefGoogle Scholar
Dennis, M., Edelstein, K., Copeland, K., Frederick, J., Francis, D.J., Hetherington, R., Blaser, S.E.Kramer, L.A., Drake, J.M., Brandt, M., & Fletcher, J.M. (2005a). Covert orienting to exogenous and endogenous cues in children with spina bifida. Neuropsychologia, 43, 976987.CrossRefGoogle ScholarPubMed
Dennis, M., Edelstein, K., Copeland, K., Frederick, J.A., Francis, D.J., Hetherington, R., Blaser, S.E., Kramer, L.A., Drake, J.M., Brandt, M.E., & Fletcher, J.M. (2005b). Space-based inhibition of return in children with spina bifida. Neuropsychology, 19, 456465.CrossRefGoogle ScholarPubMed
Dennis, M., Edelstein, K., Hetherington, R., Copeland, K., Frederick, J., Blaser, S.E., Kramer, L.A., Drake, J.M., Brandt, M., & Fletcher, J.M. (2004). Neurobiology of perceptual and motor timing in children with spina bifida in relation to cerebellar volume. Brain, 127, 12931301.CrossRefGoogle ScholarPubMed
Doyle, A.E., Willcutt, E.G., Seidman, L.J., Biederman, J., Chouinard, V.-A., Silva, J., & Faraone, S.V. (2005). Attention-deficit/hyperactivity disorder endophenotypes. Biological Psychiatry, 57, 13241335.CrossRefGoogle ScholarPubMed
Evans, B. & Waites, B. (1981). IQ and mental testing: An unnatural science and its social history. London: Macmillan Press.CrossRefGoogle Scholar
Evans, S.H. & Anastasio, E.J. (1968). Misuse of analysis of covariance when treatment effect and covariate are confounded. Psychological Bulletin, 69, 225234.CrossRefGoogle ScholarPubMed
Farah, M.J., Shera, D.M., Savage, J.H., Betancourt, L., Giannetta, J.M., Brodsky, N.L., Malmud, E.K., & Hurt, H. (2006). Childhood poverty: Specific associations with neurocognitive development. Brain Research, 1110, 166174.CrossRefGoogle ScholarPubMed
Fergusson, D.M., Horwood, L.J., & Lynskey, M.T. (1993). The effects of conduct disorder and attention deficit in middle childhood on offending and scholastic ability at age 13. Journal of Child Psychology and Psychiatry, 34, 899916.CrossRefGoogle ScholarPubMed
Fletcher, J.M., Lyon, G.R., Fuchs, L., & Barnes, M. (2007). Learning disabilities: From identification to intervention. New York: Guilford Press.Google Scholar
Flynn, J.R. (2007). What is intelligence? Beyond the Flynn Effect. New York: Cambridge University Press.CrossRefGoogle Scholar
Francis, D.J., Fletcher, J.M., Shaywitz, B.A., Shaywitz, S.E., & Rourke, B.P. (1996). Defining learning and language disabilities: Conceptual and psychometric issues with the use of IQ tests. Language, Speech, and Hearing Services in Schools, 27, 132143.CrossRefGoogle Scholar
Frazier, T.W., Demaree, H.A., & Youngstrom, E.A. (2004). Meta-analysis of intellectual and neuropsychological test performance in attention-deficit/hyperactivity disorder. Neuropsychology, 18, 543555.CrossRefGoogle ScholarPubMed
Goddard, H.H. (1917). Mental tests and immigrants. Journal of Delinquency, 2, 243277.Google Scholar
Goodenough, F.L. (1949). Mental testing. New York: Rinehart.Google Scholar
Goodman, R., Simonoff, E., & Stevenson, J. (1995). The impact of child IQ, parent IQ and sibling IQ on child behavioural deviance scores. Journal of Child Psychology and Psychiatry, 36, 409425.CrossRefGoogle ScholarPubMed
Gottfredson, L.S. (2004). Intelligence: Is it the epidemiologists’ elusive “fundamental cause” of social class inequalities in health? Journal of Personality and Social Psychology, 86, 174199.CrossRefGoogle ScholarPubMed
Gould, S.J. (1981). The mismeasure of man. New York: Norton.Google Scholar
Haier, R.J., Jung, R.E., Yeo, R.A., Head, K., & Alkire, M.T. (2004). Structural brain variation and general intelligence. NeuroImage, 23, 425433.CrossRefGoogle ScholarPubMed
Haier, R.J., Jung, R.E., Yeo, R.A., Head, K., & Alkire, M.T. (2005). The neuroanatomy of general intelligence: Sex matters. NeuroImage, 25, 320327.CrossRefGoogle ScholarPubMed
Hart, B. & Risley, T.R. (1995). Meaningful differences in the everyday experience of young American children. Baltimore, MD: Brookes.Google Scholar
Hebb, D.O. (1949). The organization of behavior. New York: Wiley.Google Scholar
Hegarty, P. (2007). From genius inverts to gendered intelligence: Lewis Terman and the power of the norm. History of Psychology, 10, 132155.CrossRefGoogle Scholar
Horn, J.L. (1998). A basis for research on age differences in cognitive capabilities. In McArdle, J.J. & Woodcock, R.W. (Eds.), Human cognitive abilities in theory and practice (pp. 5791). Mahwah, NJ: Lawrence Erlbaum.Google Scholar
Huitema, B.E. (1980). The analysis of covariance and alternatives. New York: Wiley.Google Scholar
Jensen, A.R. (1969). How much can we boost IQ and scholastic achievement? Harvard Educational Review, 39, 1123.CrossRefGoogle Scholar
Jensen, A.R. (1985). The nature of the black-white difference on various psychometric tests: Spearman’s hypothesis. The Behavioral and Brain Sciences, 8, 193263.CrossRefGoogle Scholar
Jensen, A.R. (1989). Raising IQ without raising g? A review of the Milwaukee project: Preventing mental retardation in children at risk. Developmental Review, 9, 234258.CrossRefGoogle Scholar
Jensen, A.R. & Weng, L.-J. (1994). What is a good g? Intelligence, 18, 231258.CrossRefGoogle Scholar
Jepsen, J.R.M., Fagerlund, B., & Mortensen, E.L. (in press) Do attention deficits influence IQ assessment in children and adolescents with ADHD? Journal of Attention Disorders. Doi:10.1177/1087054708322996.Google Scholar
Johnson, W. & Bouchard, T.J. (2005). The structure of human intelligence: It is verbal, perceptual, and image rotation (VPR), not fluid and crystallized. Intelligence, 33, 393416.CrossRefGoogle Scholar
Johnson, W., Jung, R.E., Colom, R., & Haier, R.J. (2008). Cognitive abilities independent of IQ correlate with regional brain structure. Intelligence, 36, 1828.CrossRefGoogle Scholar
Jung, R.E. & Haier, R.J. (2007). The parieto-frontal integration theory (P-FIT) of intelligence: Converging neuroimaging evidence. Behavioral and Brain Sciences, 30, 135154.CrossRefGoogle ScholarPubMed
Kamin, L.J. (1974). The science and politics of IQ. Potomac, MD: Lawrence Erlbaum Associates.Google Scholar
Kane, H. & Oakland, T.D. (2000). Secular declines in Spearman’s g: Some evidence from the United States. Journal of Genetic Psychology, 161, 337345.CrossRefGoogle ScholarPubMed
Kaufman, A.S. (2001). Do low levels of lead produce IQ loss in children? A careful examination of the literature. Archives of Clinical Neuropsychology, 16, 303341.CrossRefGoogle ScholarPubMed
Kelley, T.L. (1927). Interpretation of educational measurements. Yonkers-on-Hudson, New York: World Book Company.Google Scholar
Khan, S.A. & Faraone, S.V. (2006). The genetics of ADHD: A literature review of 2005. Current Psychiatry Reports, 8, 393397.CrossRefGoogle ScholarPubMed
Kornhaber, M., Krechevsky, M., & Gardner, H. (1990). Engaging intelligence. Educational Psychologist, 25, 177199.CrossRefGoogle Scholar
Kuntsi, J., Eley, T.C., Taylor, A., Hughes, C., Asherson, P., Caspi, A., & Moffitt, T.E. (2004). Co-occurrence of ADHD and low IQ has genetic origins. American Journal of Medical Genetics Part B (Neuropsychiatric Genetics), 124B, 4147.CrossRefGoogle ScholarPubMed
Lakoff, G. & Johnson, M. (1980). Metaphors we live by. Chicago, IL: University of Chicago Press.Google Scholar
Lord, F.M. (1965). A strong true-score theory, with applications. Psychometrika, 30, 239270.CrossRefGoogle Scholar
Lord, F.M. (1967). A paradox in the interpretation of group comparisons. Psychological Bulletin, 68, 304305.CrossRefGoogle ScholarPubMed
Lord, F.M. (1969). Statistical adjustments when comparing preexisting groups. Psychological Bulletin, 72, 336337.CrossRefGoogle Scholar
Lubinski, D. (2004). Introduction to the special section on cognitive abilities: 100 years after Spearman’s (1904) “‘General intelligence,’ objectively determined and measured.” Journal of Personality and Social Psychology, 86, 96111.CrossRefGoogle Scholar
Lynn, R. (1978). Ethnic and racial differences in intelligence: International comparisons. In Osbourne, R.T., Noble, C.E. & Weyl, N. (Eds.), Human variation: The biopsychology of age, race, and sex, (pp. 261286). New York: Academic Press.Google Scholar
Mathes, P.G., Denton, C.A., Fletcher, J.M., Anthony, J.L., Francis, D.J., & Schatschneider, C. (2005). The effects of theoretically different instruction and student characteristics on the skills of struggling readers. Reading Research Quarterly, 40, 148182.CrossRefGoogle Scholar
Mill, J., Caspi, A., Williams, B.S., Craig, I., Taylor, A., Polo-Tomas, M., Berridge, C.W., Poulton, R., & Moffitt, T.E. (2006). Prediction of heterogeneity in intelligence and adult prognosis by genetic polymorphisms in the dopamine system among children with attention-deficit/hyperactivity disorder: Evidence from 2 birth cohorts. Archives of General Psychiatry, 63, 462469.CrossRefGoogle ScholarPubMed
Miller, G.A. & Chapman, J.P. (2001). Misunderstanding analysis of covariance. Journal of Abnormal Psychology, 110, 4048.CrossRefGoogle ScholarPubMed
Murphy, K.R., Barkley, R.A., & Bush, T. (2001). Executive functioning and olfactory identification in young adults with attention deficit-hyperactivity disorder. Neuropsychology, 15, 211220.CrossRefGoogle ScholarPubMed
Neisser, U., Boodoo, G., Bouchard, T.J, Boykin, A.W., Brody, N., Ceci, S.J., Halpern, D.F., Loehlin, J.C., Perloff, R., Sternberg, R.J., & Urbina, S. (1996). Intelligence: Knowns and unknowns. American Psychologist, 51, 77101.CrossRefGoogle Scholar
Noble, K.G., Norman, M.F., & Farah, M.J. (2005). Neurocognitive correlates of socioeconomic status in kindergarten children. Developmental Science, 8, 7487.CrossRefGoogle ScholarPubMed
Piéron, H. (1932). Theoretical and practical aspects of intelligence. British Journal of Psychology, 22, 353358.Google Scholar
Porteus, S.D. (1917). Mental tests with delinquents and aboriginal children. Psychological Review, 24, 3241.CrossRefGoogle Scholar
Rapport, M.D., Scanlan, S.W., & Denney, C.B. (1999). Attention-deficit/hyperactivity disorder and scholastic achievement: A model of dual developmental pathways. Journal of Child Psychology and Psychiatry, 40, 11691183.CrossRefGoogle Scholar
Roll-Hansen, N. (1988). The progress of eugenics: Growth of knowledge and change in ideology. History of Science, 26, 295331.CrossRefGoogle ScholarPubMed
Rommelse, N.N., Altink, M.E., Oosterlaan, J., Buschgens, C.J., Buitelaar, J., & Sergeant, J.A. (2008). Support for an independent familial segregation of executive and intelligence endophenotypes in ADHD families. Psychological Medicine, 8, 15951606.CrossRefGoogle Scholar
Salman, M.S., Sharpe, J.A., Eizenman, M., Lillakas, L., To, T., Westall, C., Steinbach, M.J., & Dennis, M. (2006). Saccadic adaptation in Chiari Type II malformation. Canadian Journal of Neurological Sciences, 33, 372378.CrossRefGoogle ScholarPubMed
Salman, M.S., Sharpe, J.A., Lillakas, L., Steinbach, M.J., & Dennis, M. (2007). Smooth ocular pursuit in Chiari type II malformation. Developmental Medicine and Child Neurology, 49, 289293.CrossRefGoogle ScholarPubMed
Sattler, J.M. (1993). Assessment of children’s intelligence and special abilities. New York: Allyn & Bacon.Google Scholar
Schachar, R.J., Chen, S., Logan, G.D., Ornstein, T.J., Crosbie, J., Ickowicz, A., & Pakulak, A. (2004). Evidence for an error monitoring deficit in attention deficit hyperactivity disorder. Journal of Abnormal Child Psychology, 32, 285293.CrossRefGoogle ScholarPubMed
Schachar, R., Logan, G.D., Robaey, P., Chen, S., Ickowicz, A., & Barr, C. (2007). Restraint and cancellation: Multiple inhibition deficits in attention deficit hyperactivity disorder. Journal of Abnormal Child Psychology, 35, 229238.CrossRefGoogle ScholarPubMed
Schneider, W.H. (1992). After Binet: French intelligence testing, 1900-1950. Journal of the History of the Behavioural Sciences, 28, 111132.Google ScholarPubMed
Share, D.L., McGee, P.A., & Silva, P.A. (1989). IQ and reading progress: A test of the capacity notion of IQ. Journal of the American Academy of Child and Adolescent Psychiatry, 28, 97100.CrossRefGoogle ScholarPubMed
Siegel, L.S. (1992). An evaluation of the discrepancy definition of dyslexia. Journal of Learning Disabilities, 25, 618629.CrossRefGoogle ScholarPubMed
Siegel, L.S., Share, D., & Geva, E. (1995). Evidence for superior orthographic skills in dyslexics. Psychological Science, 6, 250254.CrossRefGoogle Scholar
Siegler, R.S. (1992). The other Alfred Binet. Developmental Psychology, 28, 179190.CrossRefGoogle Scholar
Spearman, C. (1904). “General intelligence,” objectively determined and measured. American Journal of Psychology, 15, 201293.CrossRefGoogle Scholar
Spearman, C. (1914). Theory of two factors. Psychological Review, 21, 105115.CrossRefGoogle Scholar
Spearman, C. (1923). The nature of “intelligence” and the principles of cognition. London: Macmillan.Google Scholar
Spearman, C. (1927). The ability of man: Their nature and measurement. New York: Macmillan.Google Scholar
Stanovich, K.E. (1986). Matthew effects in reading: Some consequences of individual differences in the acquisition of literacy. Reading Research Quarterly, 21, 360407.CrossRefGoogle Scholar
Stuebing, K.K., Fletcher, J.M., LeDoux, J.M., Lyon, G.R., Shaywitz, S.E., & Shaywitz, B.A. (2002). Validity of IQ-discrepancy classifications of reading disabilities: A meta-analysis. American Educational Research Journal, 39, 469518.CrossRefGoogle Scholar
Teasdale, T.W. & Owen, D.R. (2008). Secular declines in cognitive test scores: A reversal of the Flynn effect. Intelligence, 36, 121126.CrossRefGoogle Scholar
Terman, L.C. (1916). The Stanford revision of the Binet-Simon test. Boston, MA: Houghton Mifflin.Google Scholar
Thomson, G.H. (1916). A hierarchy without a general factor. British Journal of Psychology, 8, 271281.Google Scholar
Thomson, G.H. (1919). The hierarchy of abilities. British Journal of Psychology, 9, 337344.Google Scholar
Tupper, D.E. & Rosenblood, L.K. (1984). Methodological considerations in the use of attribute variables in neuropsychological research. Journal of Clinical and Experimental Neuropsychology, 6, 441453.CrossRefGoogle ScholarPubMed
Vellutino, F.R., Scanlon, D.M., & Lyon, G.R. (2000). Differentiating between difficult-to-remediate and readily remediated poor readers. Journal of Learning Disabilities, 33, 223238.CrossRefGoogle ScholarPubMed
Vernon, P.E. (1964). Personality assessment: A critical survey. London, Methuen.Google Scholar
Wechsler, D. (1974). Wechsler Intelligence Scale for Children-Revised. San Antonio, TX: Psychological Corporation.Google Scholar
Wechsler, D. (1991). Wechsler Intelligence Scale for Children—Third Edition. San Antonio, TX: Psychological Corporation.Google Scholar
Wilke, M., Sohn, J.-H., Byars, A.W., & Holland, S.K. (2003). Bright spots: Correlations of gray matter volume with IQ in a normal pediatric population. NeuroImage, 20, 202215.CrossRefGoogle Scholar
Wolf, T.H. (1973). Alfred Binet. Chicago, IL: The University of Chicago Press.Google Scholar
Woodcock, R.W. & Johnson, M.B. (1989). Tests of cognitive ability. New York: Riverside Publishing.Google Scholar