The article argues against using IQ as a covariate in cognitive studies of neurodevelopmental disorders. It highlights that IQ is not a suitable covariate due to its historical reification as a causal construct, its failure to meet methodological and statistical requirements, and the resulting overcorrected, counterintuitive findings. The authors contend that IQ is a volatile index of global functional outcome, influenced by genes, biology, education, and experiences, and that it is not an independent variable that can be controlled or adjusted in studies of neurocognitive function. They emphasize that IQ does not measure achievement or performance but rather aptitude and potential, which has led to its special status in cognitive research. The article provides examples from three neurodevelopmental disorders—spina bifida meningomyelocele, learning disabilities, and attention deficit hyperactivity disorder—to illustrate the issues with using IQ as a covariate. It also discusses the methodological and statistical issues associated with using IQ as a covariate, including the limitations of analysis of covariance (ANCOVA) and the potential for biased results when IQ is used to adjust for group differences. The authors conclude that IQ should not be used as a covariate in studies of neurodevelopmental disorders, as it does not provide a valid or meaningful adjustment for cognitive outcomes and can lead to misleading conclusions. Instead, they advocate for a more nuanced understanding of cognitive processes and the use of more appropriate measures in research.The article argues against using IQ as a covariate in cognitive studies of neurodevelopmental disorders. It highlights that IQ is not a suitable covariate due to its historical reification as a causal construct, its failure to meet methodological and statistical requirements, and the resulting overcorrected, counterintuitive findings. The authors contend that IQ is a volatile index of global functional outcome, influenced by genes, biology, education, and experiences, and that it is not an independent variable that can be controlled or adjusted in studies of neurocognitive function. They emphasize that IQ does not measure achievement or performance but rather aptitude and potential, which has led to its special status in cognitive research. The article provides examples from three neurodevelopmental disorders—spina bifida meningomyelocele, learning disabilities, and attention deficit hyperactivity disorder—to illustrate the issues with using IQ as a covariate. It also discusses the methodological and statistical issues associated with using IQ as a covariate, including the limitations of analysis of covariance (ANCOVA) and the potential for biased results when IQ is used to adjust for group differences. The authors conclude that IQ should not be used as a covariate in studies of neurodevelopmental disorders, as it does not provide a valid or meaningful adjustment for cognitive outcomes and can lead to misleading conclusions. Instead, they advocate for a more nuanced understanding of cognitive processes and the use of more appropriate measures in research.