diagnosing ADHD in female individuals or were more likely to diagnose it if accompanied by a comorbid disorder. However, the results did not attenuate when comorbid conditions were accounted for. Alternatively, sex-specific ascertainment biases could inflate estimated risk of ADHD in siblings of female individuals. Although female cases were more likely to be ascertained if they had an already diagnosed brother (64.6%) than vice versa, the sex-stratified results indicate that such biases cannot fully explain our results. A limitation of the analyses was restricting the sample to full siblings, which may have led to a conservative estimate of the effect size and the possibility that the results might not generalize to families with one child or half-siblings.
