The article argues against using IQ as a covariate in cognitive studies of neurodevelopmental disorders. It highlights that IQ is a volatile index of global functional outcome, influenced by genes, biology, cognition, education, and experiences. While IQ is often used to control for differences in cognitive studies, the authors argue that this practice is misguided and generally unjustified. They provide logical, statistical, and methodological arguments, supported by examples from three neurodevelopmental disorders (spina bifida meningomyelocele, learning disabilities, and attention deficit hyperactivity disorder), to show that IQ does not meet the requirements for a covariate and that using IQ as a matching variable or covariate can lead to overcorrected, anomalous, and counterintuitive findings about neurocognitive function. The historical reification of general intelligence (g) as a causal construct has contributed to the idea that IQ has special status in cognitive studies. However, g is not a stable or invariant measure and does not meet the methodological and statistical requirements of a covariate. The authors also discuss the methodological and statistical issues associated with using IQ as a covariate, including the assumptions of analysis of covariance (ANCOVA) and the potential for biased results when IQ is used to adjust for group differences. The article concludes that IQ should not be used as a covariate in neurodevelopmental studies because it is not a valid measure of cognitive function and can lead to misleading conclusions. Instead, the authors suggest that researchers should focus on understanding the role of environmental variables and the causal mechanisms underlying neurocognitive differences in neurodevelopmental disorders. The use of IQ as a covariate is inappropriate in most cases, and the field of neuropsychology needs more thoughtful use of IQ as a statistical adjustment in models of cognition. The authors emphasize the importance of developing a theoretical model of cognition before interpreting IQ scores in the context of neurodevelopmental disorders.The article argues against using IQ as a covariate in cognitive studies of neurodevelopmental disorders. It highlights that IQ is a volatile index of global functional outcome, influenced by genes, biology, cognition, education, and experiences. While IQ is often used to control for differences in cognitive studies, the authors argue that this practice is misguided and generally unjustified. They provide logical, statistical, and methodological arguments, supported by examples from three neurodevelopmental disorders (spina bifida meningomyelocele, learning disabilities, and attention deficit hyperactivity disorder), to show that IQ does not meet the requirements for a covariate and that using IQ as a matching variable or covariate can lead to overcorrected, anomalous, and counterintuitive findings about neurocognitive function. The historical reification of general intelligence (g) as a causal construct has contributed to the idea that IQ has special status in cognitive studies. However, g is not a stable or invariant measure and does not meet the methodological and statistical requirements of a covariate. The authors also discuss the methodological and statistical issues associated with using IQ as a covariate, including the assumptions of analysis of covariance (ANCOVA) and the potential for biased results when IQ is used to adjust for group differences. The article concludes that IQ should not be used as a covariate in neurodevelopmental studies because it is not a valid measure of cognitive function and can lead to misleading conclusions. Instead, the authors suggest that researchers should focus on understanding the role of environmental variables and the causal mechanisms underlying neurocognitive differences in neurodevelopmental disorders. The use of IQ as a covariate is inappropriate in most cases, and the field of neuropsychology needs more thoughtful use of IQ as a statistical adjustment in models of cognition. The authors emphasize the importance of developing a theoretical model of cognition before interpreting IQ scores in the context of neurodevelopmental disorders.