DSM-IV AD was directly assessed in the 1997–2002 interview, with new onsets coded from parent interviews. An abbreviated assessment of cannabis dependence was included in the 1997–2002 interview only. Based on published sensitivity analyses (Lynskey et al., 2002), cannabis dependence was defined as 2 or more of 4 symptoms assessed (use of larger amount/over longer period than intended; tolerance; continued use despite problems; persistent desire to cut down) within a 12-month period. We computed separately for alcohol and cannabis dependence, three dummy variables corresponding to hypothesized genetic and environmental risks, i.e., Groups 1–3, with control families (Group 4) comprising the reference group.
