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 criteria for lifetime drug dependence but had an affected identical co-twin; thus, these offspring were at low environmental risk because they did not grow up with a father with SUD, but at high genetic risk because their fathers, though unaffected, had an affected identical twin (HG-LE). Group 3 included offspring with an unaffected father whose DZ cotwin was affected; thus they were at low environmental risk but medium genetic risk (MG-LE). The lowest risk group (Group 4) comprised a random sample of offspring of male twin pairs (both MZ and DZ) in which neither had a lifetime history of drug dependence.
