Genetic risk for externalizing problems was assessed by constructing polygenic scores (PGS), which are aggregate measures of the number of risk alleles individuals carry, each weighted by effect sizes from GWAS summary statistics. We used PRS-CS auto version (Ge et al. 2019) to calculate the externalizing polygenic scores. This approach employs a Bayesian regression and continuous shrinkage method to correct for the non-independence among nearby SNPs in the genome (i.e. linkage disequilibrium, LD). In keeping with the recommendations of the PRS-CS developers, we limited the SNPs included in the PGS to HapMap3 SNPs that overlapped between the original GWAS summary statistics and the LD reference panel (1000 Genomes Phase III reference panel).
