Background: Working memory, inhibition, and expressive language are often impaired in ADHD and many\nchildren with ADHD have lower IQ-scores than typically developing children. The aim of this study was to test\nwhether IQ-score influences associations between ADHD symptoms and verbal and nonverbal working memory,\ninhibition, and expressive language, respectively, in a nonclinical sample of preschool children.\nMethods: In all, 1181 children recruited from the Norwegian Mother and Child Cohort Study were clinically\nassessed at the age of 36 to 46 months. IQ-score and working memory were assessed with subtasks from the\nStanford Binet test battery, expressive language was reported by preschool teachers (Child Development Inventory),\nresponse inhibition was assessed with a subtask from the NEPSY test, and ADHD symptoms were assessed by\nparent interview (Preschool Age Psychiatric Assessment).\nResults: The results showed an interaction between ADHD symptoms and IQ-score on teacher-reported expressive\nlanguage. In children with below median IQ-score, a larger number of ADHD symptoms were more likely to be\naccompanied by reports of lower expressive language skills, while the level of ADHD symptoms exerted a smaller\neffect on reported language skills in children with above median IQ-score. The associations between ADHD\nsymptoms and working memory and response inhibition, respectively, were not influenced by IQ-score.\nConclusions: Level of IQ-score affected the relation between ADHD symptoms and teacher-reported expressive\nlanguage, whereas associations between ADHD symptoms and working memory and response inhibition,\nrespectively, were significant and of similar sizes regardless of IQ-score. Thus, in preschoolers, working memory and\nresponse inhibition should be considered during an ADHD assessment regardless of IQ-score, while language skills\nof young children are especially important to consider when IQ-scores are average or low.
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