We tested two specific hypotheses for the male bias in ADHD: first, that sex-specific genetic heterogeneity may affect prevalence rates via clinical heterogeneity in ADHD diagnosis by sex, and second, that female individuals affected with ADHD may carry an increased burden of genetic risk variants compared with affected male individuals. We analyzed autosomal common variant data from the largest available ADHD case-control GWAS sample (n = 55,374 individuals) and Swedish population register data (n = 1,952,542 individuals). We demonstrated a high level of genome-wide autosomal genetic correlation for ADHD across sex and found no increase of polygenic burden in affected female individuals compared with affected male individuals. However, in register-based data, we observed that siblings of female individuals with ADHD are at increased risk for ADHD compared with siblings of affected male individuals. The results also suggested that female individuals diagnosed with ADHD may be at an especially high risk for certain comorbid developmental conditions.
