Based on the AD and DD statuses of the father and his co-twin, offspring were classified into one of seven groups representing different levels of genetic and environmental risk for the offspring. Paternal substance dependence was the most proximal indicator of risk, and thus took precedence over uncle status. DD was also presumed to represent greater risk than AD and thus was given precedence in group assignment. Offspring whose father had a history of DSM-III-R illicit drug dependence were assigned to risk group D1 (regardless of AD status); offspring whose father had no history of DD but had a history of DSM-III-R AD were assigned to risk group A1. Offspring in these groups are, on average, at high genetic and high environmental risk. Additional risk groups were based on the twin pair’s zygosity and the uncle’s DD/AD status when the father was not DD or AD. Offspring whose uncle was an MZ co-twin of their father were assigned to group D2 if the co-twin was DD, and to group A2 if the MZ uncle was AD (these offspring are at
