Past epidemiological studies have documented the extensive comorbidities of psychiatric disorders at the phenotype level. Due to limitations inherent to observational studies, understanding whether a phenotypic correlation is potentially causal or if it results from reverse causation or confounding is generally difficult or impossible. Genetic studies now offer complementary strategies. We can readily assess whether a phenotypic association between psychiatric disorders or between a psychiatric disorder and a risk factor is mirrored by a common variant genetic correlation. This can be done using GWAS summary statistics. If the genetic studies are sufficiently large, it is also possible to apply Mendelian randomization to evaluate the potential causality of the association (49).
