To test for evidence of increased risk for ADHD in siblings of female individuals with ADHD, we used Swedish population data to select sibling pairs where at least one child had ADHD. The results showed that siblings of female individuals diagnosed with ADHD are at higher risk for ADHD than siblings of affected male individuals (OR [CI] = 1.14 [1.11–1.18], p = 1.5E-15); results stratified by comparison sibling sex were similar for male individuals (OR [CI] = 1.18 [1.13–1.23], p = 1.8E-13) and female individuals (OR [CI] = 1.09 [1.01–1.16], p = .017). Covarying for presence of ASD, congenital malformations, and ID in the proband did not affect the overall results (OR [CI] = 1.15 [1.11–1.19], p = 7.3E-16).
