We report a lower h2SNP for ADHD (h2SNP = 0.14) than estimated previously (h2SNP = 0.22). This is driven by a lower h2SNP in the PGC and deCODE cohorts compared to iPSYCH. Different ascertainment and diagnostic strategies and designs among PGC cohorts could decrease the h2SNP, while a lower effective sample size43 in Iceland, and thus fewer recent variants, might bias h2SNP downwards in the deCODE cohort44.
