The interview was a modified version of the Semi-Structured Assessment on the Genetics of Alcoholism (Bucholz et al., 1994) and the smoking section was modified from the Composite International Diagnostic Interview (Cottler et al., 1991). Analyses reported here included twins aged 15–21 at the time of the telephone diagnostic interview (N=1953 female twins and N=1324 male twins) for whom we had adequate information on nicotine withdrawal in smokers (defined here as smoking 100 or more cigarettes lifetime). From this pool, 21.4% (N=417, mean age= 17.8, standard deviation (SD) =1.6) of girls and 21.5% (N=285, mean age = 18.9, SD=1.7) of boys reported smoking 100 or more cigarettes lifetime and a 24-hour period of quitting or cutting down on cigarettes, and were queried about the experience of nicotine withdrawal symptoms during this time. Over 90% of the boys (91.0%) and girls (92.7%) who reported smoking 100 or more cigarettes lifetime reported smoking daily for three weeks or more. Nicotine withdrawal was also assessed in some of the adolescents who smoked 20–99 cigarettes lifetime (N~200), but because this threshold of smoking was not comparable to our adult population (see below), we did not include these individuals in the analyses.
