Results for first cigarette use are shown in Table 6. For all models, parent and cotwin alcohol and cannabis dependence could be equated in Groups 2 and 3, and in Group 1 from age 9 onwards. In the fully adjusted Model III, through age 8, offspring of cannabis dependent parents (Group 1b) were at 7.30 times increased risk of early smoking, compared to controls (Group 4). However, offspring of alcoholic parents (Group 1a), were at 74% reduced risk of early smoking, also through age 8. From age 9 onwards, risk to offspring of alcoholic or cannabis dependent parents (Group 1c) was nonsignificant. Offspring of unaffected parents with a substance dependent identical cotwin (Group 2) were at 2.22 times greater risk of early smoking, with offspring of unaffected parents with a fraternal cotwin who is substance dependent (Group 3) at little to no risk relative to controls. Parental separation uniquely predicted 77% higher likelihood of early smoking without significant age interaction.
