Latent (unmeasured) levels of familial and environmental risk to offspring were assigned to one of four categories based on each twin pair’s (father and uncle of offspring) lifetime drug dependence status and zygosity (MZ or DZ). The highest risk category (Group 1) comprised offspring of MZ or DZ fathers who had a history of drug dependence, regardless of co-twin status for drug dependence, or who had a history of alcohol dependence, if the co-twin had a history of drug dependence. This was termed the high-genetic, high-environmental (HG-HE) risk group, since offspring were at elevated genetic risk by virtue of having a father with SUD and also at elevated environmental risk by virtue of experiencing a high risk rearing environment via their affected parent (Hartman et al., 2006, Hussong et al., 2008, Jacob et al., 2003, Thompson et al., 2008). Groups 2 and 3 were offspring of MZ (Group 2) and DZ (Group 3) twins, in which 100% (MZ) and 50% (DZ) of genes are shared between the twin fathers. Offspring in Group 2 had fathers who did not themselves meet
