We must note assumptions of heritability analyses including random mating and equal environments (71). On one hand, violations of the random-mating assumption would result in downwardly biased estimates of coheritability (72). On the other hand, violations of the equal environment assumption would result in upwardly biased estimates of coheritability (72), though there is evidence that this bias, when present, is modest (73,74). An additional limitation of heritability analyses is that the statistical power required to parse the role of overlapping genetic and shared environmental factors is substantial, and it is beyond the scope of the current analysis. However, given that we found little evidence that shared environment contributes to the correlation of GMV and alcohol use, shared environment is an unlikely confound.
