Our finding of a substantial rg SNP of +0.43 between schizophrenia and major depressive disorder is notable and contrary to conventional wisdom about the independence of familial risk for these disorders. However, because major depressive disorder is common, even a high genetic correlation implies only modest incremental risk. Assuming the disorder risks and heritabilities for schizophrenia and major depressive disorder given in Table 1, then the genetic correlation between them of 0.43 predicts increased risk of major depressive disorder in first-degree relatives of probands with schizophrenia compared to first-degree relatives of control probands (λSCZ,MDD) of 1.6. In fact, meta-analysis of five interview-based research studies of families are broadly consistent with our results (λSCZ,MDD = 1.5, 95% confidence interval (CI) = 1.2–1.8; Supplementary Table 9), suggesting that familial coaggregation of major depressive disorder and schizophrenia reflects genetic effects rather than resulting from living in a family environment that includes a severely ill family member. If replicated by future work, our empirical molecular genetic evidence of a partly shared genetic etiology for schizophrenia and major depressive disorder would have key nosological and
