manic symptoms than the other major bipolar disorder subtype) and an increased risk for bipolar disorder I in relatives of individuals with ADHD12. These findings could mean that the familial link between the two disorders is mediated by environmental risk factors or that shared genetic factors are not part of the common allelic spectrum. Alternatively, the etiological link between ADHD and bipolar disorder might be limited to bipolar disorder I or early-onset bipolar disorder12, which, therefore, is difficult for us to detect. Our finding of genetic overlap between ADHD and major depressive disorder is consistent with evidence from studies showing increased rates of ADHD in the families of depressed probands and increased rates of depression in families of probands with ADHD12,13.
