To investigate the validity of EXT, in each of these two samples, we generated a phenotypic externalizing factor by fitting a factor model to phenotypic data corresponding to the seven discovery phenotypes (Extended Data Fig. 9 and Supplementary Table 27). Controlling for age, sex, and ten genetic principal components, the genome-wide polygenic score was associated with the phenotypic factor in both data sets (βAdd Health = 0.33, 95% CI: 0.30–0.36, ΔR2 = 10.5%; βCOGA = 0.30, 95% CI: 0.27–0.34, ΔR2 = 8.9%; Figure 3A and Supplementary Table 28). The variance explained by the EXT polygenic score (ΔR2 ~ 8.9–10.5%) is commensurate with many conventional variables used in social science research, including parental socioeconomic status, family income or structure, and neighborhood disadvantage/disorder43–45. Next, as further quasi-replication, we created a polygenic score using only the 579 EXT SNPs (this score was only used for this quasi-replication exercise), and also this polygenic score was found associated with the phenotypic externalizing factor, explaining ~3–4% of the variance (βAdd Health = 0.20, 95% CI: 0.17–0.23, ΔR2 = 4.1%; βCOGA = 0.17, 95% CI: 0.13–0.20, ΔR2 = 3.0%).
