Heritability estimates for the resulting GMM trajectories were derived using structural equation modeling in Mplus.34 To ascertain the heritability of trajectory group membership, we used the continuous posterior probabilities that each individual belongs to each trajectory group. These posterior probabilities are continuous and available for every person at every time point. For each of the six trajectories, we used a two-group (monozygotic [MZ]–dizygotic [DZ]) twin model for continuous outcomes, where factors represent the additive genetic (A), common environmental (C), and unique environmental (E) components. Finally, mean psychopathology, personality, and cognitive ability values for each trajectory group were examined using rank-order correlations. This approach examines whether the external correlates of alcohol use change monotonically with trajectory rank.
