In Model I, offspring of substance dependent fathers (Group 1) were at a 28% increased hazard of alcohol use onset, compared to controls (Group 4), see Table 2. Risk to offspring of unaffected fathers with a DD/AD MZ or DZ co-twin (Groups 2 and 3, respectively) was slight and non-significant. In post-hoc tests, risk to Group 1 was greater than Group 2, with non-significant differences between the other groups. In Model II, risk to Group 1 offspring was again increased, relative to Group 4 (HR = 1.24), although differences among Groups 1–3 were non-significant in post-hoc tests. A violation of the proportional hazards assumption was observed for parental separation, necessitating modeling of an age interaction. Parental separation was associated with a 3.71 times higher hazard of alcohol initiation through age 12, and with approximately a 62% increased hazard over ages 13–15, with no differences from age 16 onwards. In Model III, controlling for family background and offspring characteristics, Group 1 offspring were at a 29% increased hazard of alcohol use, with differences among Groups 1–3 non-significant. In addition, parental separation
