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 is, we believed that answers from former (or current) regular smokers about when they began regular smoking would be more reliable than answers from people who never became regular smokers about when they first tried cigarettes. Further research is needed to differentiate between the different genetic processes that may be influencing the results presented here. Finally, although this is a national study of twins, it is possible that these findings are unique to this sample. Although we show convergence in these findings using multiple methods, replication of these findings using data obtained from different samples across similar cohorts is necessary to fully validate our results.
