P-value threshold based polygenic scores (GPS) were calculated in ALSPAC using the S4S GWAS results. Prior to GPS derivation, we identified markers present in both the S4S and ALSPAC genetic data with MAF > 0.01 and INFO > 0.5. We pruned this list using the clumping function in PLINK 1.9 (Chang et al., 2015) to identify independent loci. Clumping was conducted based on the European subsample of the 1000 Genomes reference panel, in two stages: the first used an r2 threshold of 0.5 and a range of 250 kb; the second used an r2 threshold of 0.1 and a range of 5 mb. Markers selected were used to derive GPS for ALSPAC participants, employing a series of p-value thresholds as is customary. We derived GPS based on the Z-scores (and associated p-values) from the METAL meta-analyses as well as based on EUR-specific SNP regression coefficients (and associated p-values), given the possibility that the latter might be more predictive of outcome in the European ALSPAC sample. In R (v3.2.3), we tested whether GPS were associated with alcohol outcomes (Consumption, Problems, or
