I was aware of ADHD's general heritability, and the general lack of any definitive GWAS. Brikell's study is interesting, thanks for pointing it out - but the full study is sadly behind a paywall. Bright side: the part outside the paywall summarizes the methods section (which looks sound). But the statistical methods they were using for analysis and raw data are hidden from general view. They do mention the number of participants, but without knowing the variance there's no way to work out the p value.