Expanding and refining phenotypes would further elaborate the results of current study. Specifically, MD was the only internalizing disorder included in this study. Although a class for MD contains a set of SNPs distinctive from the AD+ASP class, the MD class alone may not fully represent internalizing disorders. Identification of a class of SNPs for multiple internalizing disorders, such as anxiety disorder or phobia in addition to MD, would confirm the cluster of SNPs corresponding to the internalizing genetic factor. In addition, twin studies have indicated that the externalizing and internalizing genetic factors may not be completely exclusive in influencing each disorder. Externalizing and internalizing genetic factors may be correlated (Kendler et al. 2011b), or cross loadings may exist for a single disorder from both internalizing and externalizing genetic factors (Kendler et al. 2003). Moreover, it is possible that subtypes of a disorder may have different genetic factors as indicated by, for example, studies on internalizing subtypes of AD (Edwards et al. 2014; McCarty et al. 2012; Needham 2007). However, these possibilities were not considered in this study.
