rank will change monotonically with heritability estimates and various external correlates of alcohol use, including personality traits, cognitive abilities, and psychiatric disorder symptoms. If trajectory groups represent severity gradations rather than distinct subgroups, then the external correlates of alcohol use should change monotonically with trajectory rank.18 Similarly, because heritability estimates are higher in more severe alcohol use groups, the heritability of group membership should also increase across the trajectories.19 On the other hand, if alcohol trajectories are qualitatively distinct, there will be little evidence for this stepwise pattern of genetic and external continuity.
