We fit a piecewise linear growth model to evaluate different growth trajectories for the adolescent (≤ 18 years) and adult (≥ 19 years) periods by estimating different intercepts and slopes for each period. The transition from adolescence to adulthood is a period of considerable theoretical interest with important developmental changes associated with the attainment of adult status. Therefore, we modeled the transition from adolescence to adulthood as the discrepancy between the expected values at 19 based on the growth trajectories of adolescence (data from 14 to 18 years) and young adulthood (data from 19 to 25 years). As seen in Figure 1, the mean trajectory of drunkenness shows a steadily increasing trend up to age 18, a substantial increase in the average response between 18 and 19 (referred to here as the “jump”), and a leveling off in drunkenness that occurs after that period. Differences in each of these parameters (slope of drunkenness across adolescence, transition point (“jump”), and slope of drunkenness across young adulthood) were tested by genotype. The model was fit in the framework of hierarchical linear modeling
