This study followed a preregistered analysis plan (https://osf.io/4f5x8). The regression analyses were cross-sectional and conducted in R (R Core Team, 2021). First, to test Hypothesis 1, the externalizing behavior score was regressed on the EXT PGS. To test Hypothesis 2, the externalizing behavior score was regressed on P3 amplitude. Hypothesis 3 was tested by regressing P3 amplitude on EXT PGS. We then performed mediation analyses (Hypothesis 4) to determine the indirect effect of P3 amplitude on the association between EXT PGS (independent variable) and the externalizing behavior score (dependent variable). All analyses included relevant covariates as applicable (top 10 ancestry principal components, age, sex, etc.).
