Familial factors inflate the estimated effects of genetic liability to ADHD on behavioural outcomes

Greater genetic liability to ADHD predicts worse smoking, alcohol use, depressive symptoms, and educational/cognitive outcomes in population analyses, but these effects attenuate in within-sibship family analyses.

• Why it matters: Conventional genetic studies cannot separate an individual’s ADHD liability from shared family environment, risking over-attribution of behavioural and health harms to the person’s own ADHD.
• What they did: Using up to 22 genome-wide significant ADHD variants (38,691 cases, 186,843 controls), the study estimated effects on 16 outcomes and compared population estimates with within-sibship analyses in up to 164,220 siblings.
• The result: For depressive symptoms, the effect dropped from 0.059 SD (95% CI 0.039, 0.080) to 0.002 SD (-0.052, 0.055), and for age at first child from -0.045 SD (-0.058, -0.033) to 0.003 SD (-0.040, 0.047), with significant design differences (p=0.048 and p=0.034).