Across four criteria, our results suggest that the development of alcohol use is continuous rather than categorical. First, for all analyses, each added trajectory improved the fit of the model. This finding suggests that the analytic method is trying to model an underlying continuum and gets closer with each successive trajectory. Second, the distribution of individuals across the trajectory arcs was relatively normal, with most individuals grouped into moderate trajectories and fewer individuals grouped in higher and lower trajectories. Third, the rank-order of individuals was stable throughout development—the trajectory arcs did not cross or show qualitatively distinct patterns of growth. And fourth, genetic and external evidence suggests that these arcs reflect ordinal points on a severity continuum—the heritability and external correlates of alcohol use change monotonically across the trajectory ranks. These findings support the developmental continuity of alcohol use and challenge the trajectory group approach. Of course, future research must determine whether such findings generalize beyond our sample of mostly White twins from Minnesota, and whether evidence of developmental continuity extends beyond the age of 29.
