The EXT PGS was not significantly associated with P3 amplitude. Consistent with this finding, the indirect effects of P3 amplitude on the association between the EXT PGS and externalizing behavior from all mediation models were not significant. When the externalizing behavior score was simultaneously regressed on the EXT PGS and P3 amplitude (and relevant covariates), the magnitude of association was similar to the separate models for both the EXT PGS and P3 amplitude (Table 4). For example, when the externalizing score was regressed on the EXT PGS and P3 amplitude in EA young adults, both independent variables remained significantly associated with externalizing behavior (βEXT PGS = 0.09 [95% CI 0.04–0.13], βP3 = −0.05, [95%CI −0.09 to −0.01]), with standardized beta values of the same magnitude as those reported in Table 3, when each independent variable was modeled separately. Therefore, the EXT PGS and P3 amplitude account for unique and independent variation in externalizing behavior in this sample.
