Our results should be interpreted in the context of four potentially important methodological limitations. First, any artifacts that make SNP genotypes more similar between cases than between cases and controls could inflate estimates of SNP-based heritability18, but to a much lesser extent for SNP-based coheritability. Second, the sample sizes varied considerably across the five disorders. Although hSNP2 values are expected to be unbiased, estimates from smaller samples are accompanied by larger standard errors, blurring their interpretation. Third, although applying similar diagnostic criteria, the clinical methods of ascertainment and the specific study protocols, including which specific interview instruments were employed, varied across sites. We cannot now determine the degree to which our results might have been influenced by between-site differences in the kinds of patients seen or in their assessments. Fourth, by combining samples from geographic regions, contributions from less common associated variants specific to particular populations are diluted compared to what would have been achieved if the same sample size had been ascertained from a single homogeneous population.
