followed: the first portion of the slope reflected the change in heavy episodic drinking from adolescence (age 15.5) to early adulthood (age 21.5), and the second portion of the slope reflects adult trajectories of heavy episodic drinking, from age 23.5 to age 27.5. The intercept was set at age 15.5. Given the possibility of sex differences in genetic heritability estimates for alcohol use problems (King et al., 2005), analytic models were tested separately for males and females, resulting in six total models tested. Hence, direct statistical comparisons (e.g., mean differences) between males and females were not directly tested in the current study. Upon selection of the best fitting model, covariates were included into a hierarchical poisson regression model. In the initial model step, intercept and slopes were estimated. In the second model, covariates (i.e., school substance use, parental knowledge), close friend substance use and PRS were added. In the final (fully saturated) model, cross-products between PRS and close friend substance use variables were included.
