The co-morbidity of externalizing disorders is largely attributed to shared genetic liability (Dick, Viken, Kaprio, Pulkkinen, & Rose, 2005; Waldman, Poore, van Hulle, Rathouz, & Lahey, 2016) and the heritability of a general externalizing factor has been estimated to be quite high (h2 0.81–0.84; Krueger et al., 2007; Young, Stallings, Corley, Krauter, and Hewitt, 2000). GWAS of individuals of European ancestry (EA) have been recently used with great success to identify many genetic variants – typically single nucleotide polymorphisms (SNPs) – that are associated with complex traits including ADHD and alcohol use. Genomic structural equation modeling (gSEM) is a multivariate method developed for analyzing the joint genetic architecture of complex traits (Grotzinger et al., 2019), and was recently applied to data from GWAS of externalizing behaviors. This model used data from large GWAS (N > 50 000) available for seven externalizing phenotypes (ADHD, problematic alcohol use, lifetime cannabis use, age at first sexual intercourse, number of sexual partners, risk-taking, and lifetime smoking initiation). The model included data from 1.5 million EA individuals and indicated a single genetic factor underlying the
