Twin studies of the genetic architecture of comorbid psychiatric disorders suggest considerable genetic overlap across psychiatric conditions. Analyses of the covariance structure of comorbid psychiatric disorders have suggested a hierarchical factor structure (Krueger 1999; Krueger et al. 2001; Krueger and Markon 2006), with two primary factors comprising internalizing and externalizing disorders. Internalizing disorders encompass major depression, dysthymia, generalized anxiety disorder, phobias, and panic disorder, and externalizing disorders encompass alcohol dependence, drug dependence, conduct disorder, and antisocial personality disorder (Krueger 1999). Behavioral genetic studies using twin data indicate that genetic factors largely account for shared variation within internalizing and externalizing disorders, respectively (Kendler et al. 2003; Kendler et al. 2011a; Krueger et al. 2002). In an effort to take into account the shared genetic epidemiology of internalizing or externalizing disorders, investigators have sometimes created composite internalizing or externalizing phenotypes for use in genetic analyses (Benke et al. 2014; Derringer et al. 2015; Dick et al. 2008; McGue et al. 2013). Genome-wide association studies (GWAS) results of individual psychiatric outcomes have also been used to examine genetic relationships between multiple disorders. Such
