either as latent variables in twin and family studies or as genuine measured genes in molecular genetic studies. When genetic effects are modeled latently, the contribution of gene effects is inferred based on observed correlations between people with different degrees of sharing across genes or the environment (5). These correlations are used to study whether the heritability is the same in different groups. Meanwhile, molecular genetic studies focus mostly on one specific gene of interest, rather than the aggregate effect of genes influencing a trait. Despite awareness of the importance of gene-environment interactions in tobacco use, studies available on the subject are currently limited. Evidence from twin studies have predominantly focused on the importance of genetic factors influencing cigarette initiation, as it relates to family environment, school environment, neighborhood characteristics, and religion, while molecular genetic studies of social policy and the environment have investigated whether genetic influences on initiation, daily smoking, and cessation are moderated by social policy and the environment. All studies discussed in this section still await replication.
