Intellectual achievement and cognitive functioning in children and adolescents with ADHD-affected siblings: A systematic literature review and meta-analysis

BACKGROUND: Emerging literature suggests that neurocognitive traits commonly observed in children and adolescents with attention-deficit/hyperactivity disorder (ADHD) are present, albeit to a lesser degree, in their non-ADHD siblings. This meta-analytic review aimed to quantitatively summarize intellectual and cognitive performance differences between children and adolescents without ADHD and both their ADHD-diagnosed siblings and unrelated peers without ADHD. METHODS: June 2025. Meta-analyses were conducted using three-level hierarchical models to account for the dependency among effect sizes derived from the same study. Between-group differences in intellectual and cognitive scores were quantified using standardized mean differences (SMD). Moreover, we assessed the robustness of the findings by testing for potential small-study effects. RESULTS: Based on 404 unique effect sizes derived from 32 studies, involving a total of 8873 participants, the current results demonstrate that the overall cognitive-intellectual performance of siblings surpasses their ADHD-affected siblings (SMD = 0.30, 95 % CI [0.23, 0.38]), but falls short of unrelated peers (SMD = -0.28, 95 % CI [-0.35, -0.21]). This suggests that non-ADHD siblings exhibit an intermediate neurocognitive profile between their ADHD-affected siblings and peers without ADHD. Notably, we observed this pattern of results consistently across the major cognitive categories examined (intellectual functioning, attention and perceptual-temporal processing, memory, and executive functions). Importantly, no evidence of small-study effects was found. CONCLUSION: The current findings provide robust support for cognitive-intellectual vulnerabilities in siblings of ADHD-diagnosed children and adolescents, possibly accounted for by genetic inheritance, shared environmental influences, and gene-environment interactions.