This study examined the patterns of clustering of SNP association results by applying LPA to genome-wide association results across AD, ASP, and MD. Conditional on considering models consistent with expectations from twin studies regarding the number of classes, the results indicated that the classifications of SNPs based on their patterns of associations broadly reflected externalizing and internalizing genetic factors (Kendler et al. 2003; Kendler et al. 2011a; Krueger et al. 2002). Especially in the 3-class model, SNPs classified into the AD+ASP class, which was characterized by stronger associations with two externalizing phenotypes (AD and ASP) but not with MD, an internalizing phenotype, may represent a cluster of SNPs that affect externalizing disorders broadly. Thus, this cluster of SNPs may represent part of the molecular genetic basis for the shared latent genetic factor observed in twin studies. The MD class, which was distinguished from the AD+ASP class by relatively stronger association with MD only, more likely represents a cluster of SNPs that may reflect the internalizing genetic factor.
