First, we expect that analytic fit statistics will continue to indicate that a greater number of trajectories always fit the data better than a lesser number of trajectories (suggesting that each additional trajectory brings the set closer and closer to approximating the underlying developmental continuum). Second, we expect that the distribution of individuals across the trajectory arcs will be relatively normal; that is, the growth mixture modeling will disperse the underlying continuum of alcohol use into a “rainbow” of developmental arcs, ranked from lower levels of use to higher levels of use, with most individuals grouped into moderate trajectories and fewer individuals grouped in higher and lower trajectories. Third, we expect that the rank-order of individuals will be stable throughout development—that trajectory arcs will not cross or show qualitatively distinct patterns of growth. And fourth, we expect that alcohol trajectory 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
