Because the samples for the current project were from two separate offspring of twins design studies, the sampling design variables for these projects were included in all analyses. The sampling design variable was a 7-level variable based on father and co-twins alcohol dependence (AD) and drug dependence (DD) status and zygosity. Level 1 consisted of offspring born to fathers with DD with and without AD. Father DD was highly comorbid with AD and therefore considered together in DD fathers. Level 2 offspring were born to unaffected MZ twins whose co-twin had DD with and without AD. Level 3 offspring were born to unaffected DZ twin fathers whose co-twin had DD with and without AD. Level 4 offspring were born to fathers with AD. Level 5 offspring had unaffected MZ twin fathers whose co-twin had AD. Level 6 offspring had unaffected DZ twin fathers whose co-twin had AD, and Level 7 offspring were born to MZ and DZ twins without DD and AD.
