Epidemiological analyses also showed that siblings of female individuals diagnosed with ADHD were at higher risk for being diagnosed with ADHD than siblings of diagnosed male individuals. This confirms results from previous family studies 29, 30, 31 indirectly supporting the hypothesis that female individuals require a greater burden of genetic risk to manifest ADHD. Although these analyses do not distinguish between genetic and shared environmental effects, twin studies have consistently demonstrated high heritability of ADHD with typically nonsignificant contributions from shared environmental factors (2), suggesting that this effect is most likely to be genetic in origin. However, the effect sizes were not large (OR = 1.09–1.18), suggesting that any increased burden of inherited genetic variation might be only a small contribution to the sex bias in ADHD prevalence. These results could also occur if clinicians had a higher threshold for 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
