The magnitude of the associations between polygenic scores and alcohol problems was fairly consistent across the range of selected p-value thresholds, and accounted for, on average, 0.63% of the variance in alcohol problems (range 0.55%–0.70%). To be sure that our effects were not driven by non-independence within the sample, we re-ran the association analyses after randomly dropping one member from each twin pair (n = 634) and found the same pattern of results. This is substantially lower than the estimate (derived from the pattern of MZ and DZ twin correlations in the same sample) that additive genetics effects account for 34% of the variance in alcohol problems. We note, however, that heritability estimates derived from twin models and the variance accounted for by a polygenic score are not directly comparable. Polygenic scores are composed of SNPs across a range of p-value thresholds, and thus their genetic informativeness is likely to be somewhere between a polygenic risk score based on genome-wide significant SNPs and SNP heritability as derived through methods that estimate the variance explained by genome-wide markers (e.g., GCTA; [44]).
