Third, we do not differentiate between non-smokers who have never tried a cigarette and those who have tried cigarettes but never progressed to regular smoking. This distinction is important because different genes are believed to underlie smoking initiation and regular smoking (Madden et al. 1999). The initiation of smoking is influenced by impulsivity, whereas regular tobacco use is linked more strongly to individual differences in nicotine metabolism (Lerman et al. 1999; Pianezza, Sellers, and Tyndale 1998; Gu et al. 2000). The genetic factors responsible for these more proximate phenotypes are quite different from one another and therefore may respond very differently to the same environmental forces. In the same manner, the social moderation of genetic factors does not mean that the same genes are being turned off or turned on across time. Although examining these molecular processes would clarify this complex picture, it is not possible to perform this kind of analysis with these data. We chose to focus on regular smoking primarily because we felt that it might have greater reliability for the older respondents in our survey. That
