The cognitive manifestations and prevalence rates of co-occurring ADHD and RD across development: A systematic review and meta-analysis

Attention deficit hyperactivity disorder (ADHD) and reading difficulty (RD) frequently co-occur, yet it is unclear whether their co-occurrence and cognitive profiles remain stable across the lifespan. This review examined whether the rates and cognitive manifestations of ADHD+RD change from childhood to adulthood. Embase, MEDLINE, PsycINFO, Web of Science, CINAHL, and PubMed were searched. Studies were included if they reported the proportion of children or adults with ADHD+RD, or assessed cognitive performance in at least one of the following domains: attention, cognitive flexibility, working memory, inhibition, general memory, planning, problem-solving, processing speed, or vocabulary. Random-effects proportional meta-analyses synthesised co-occurrence rates across child and adult studies, which were compared using a z-test. Random-effects meta-analyses of standardised mean differences synthesised pooled effect estimates across cognitive domains for children and adults, which were compared using an independent samples t-test. This review was pre-registered on PROSPERO (number: CRD42023463062). Of the 3,981 records identified, 153 studies were eligible: 98 examined prevalence, and 69 examined cognition. Pooled ADHD+RD prevalence was 0.23 (95% CI [0.20, 0.27]) in children, and 0.23 (95% CI [0.12, 0.40]) in adults. Pooled cognitive performance was -1.11 (SE = 0.09, 95% CI [-1.28, -0.95]) for children, and -0.70 (SE = 0.11, 95% CI [-0.91, -0.49]) for adults, with no significant child-adult differences across domains. Heterogeneity remained high across all syntheses (I2 > 75%). The findings indicate comparable ADHD+RD co-occurrence rates and cognitive performance across development, have implications for the assessment, diagnosis and support of ADHD+RD, and highlight several methodological considerations for future research.