It is worth commenting on the magnitude of the observed effects. The variance accounted for by genome-wide polygenic scores for alcohol problems and related substance use behaviors remains very modest (< 1% of the variance)39. We note that in genetic nurture analyses for tobacco use, Saunders et al.51 reported indirect effect sizes of a magnitude comparable to our findings for alcohol phenotypes. The modest predictive ability of the alcohol problems polygenic score also cautions against overinterpreting the parent-of-origin effects, particularly in view of the observation that the maternal and paternal effects were typically of the same magnitude and direction of effect.
