The lack of overlap between ADHD and ASD is unexpected and is not consistent with family and data linkage studies, which indicate that the two disorders share genetic risk factors5,6,58,59. Some rare copy number variants are seen in both disorders16. As noted above, the use of pseudocontrols for many of the ASD and ADHD cohorts may affect all results for these disorders. Ideally, we would investigate the impact of pseudocontrols, given the hierarchical diagnostic system (autism but not autism spectrum is an exclusion criterion for most ADHD data sets), on estimates of SNP-based coheritability, but the small ASD sample size prohibits such analyses. We also found no overlap between ADHD and bipolar disorder, despite support from meta-analysis results of an increased risk for ADHD in relatives of individuals with bipolar disorder I (a subtype of bipolar disorder with more extreme 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
