The present study involved analyses of data from twin fathers and from diagnostic telephone interviews (Bucholz et al. 1994) with biological mothers and their offspring in two complementary offspring-of-twins projects conducted from 2001–2004. All offspring were born to twin fathers from the VETR. Data from a 1992 interview with the twin fathers (Tsuang et al 1996) permitted classifying twin fathers as alcohol dependent, drug dependent or both. From these data, families in the offspring of twins projects were selected if the twin fathers were concordant or discordant for alcohol dependence (Project 1), and in Project 2 families were selected if twin fathers were concordant or discordant for illicit drug dependence (DD). Both studies included unaffected twin pairs and their families as controls. We combined data from both offspring of twins projects to increase our total sample size. This was possible because both studies used similar survey instruments. The present data set was created by using data from the most recent interview if subjects had participated in both Projects 1 and 2. In the present study we analyzed data from 1,107
