Both twin/family and molecular genetic studies provide preliminary evidence that gene-environment interactions have differential effects on cigarette use over the course of development. Twin and family studies demonstrate that the relative contributions of genetic and environmental factors to cigarette use changes across time from adolescence, when most smokers initiate cigarette use, through adulthood, when many smokers have established patterns of cigarette use. Familial and environmental factors contribute to whether individuals initiate cigarette use. However, as individuals move from initiation to more established patterns of use, the importance of common environmental factors decreases while the influence of genes increases. As the contribution of genes to cigarette use increases, the influence of environmental factors does not go away, but rather, environmental factors begin playing a different, but still important role – that is, as a moderator of the influence of genetic susceptibilities (105). This implies the presence of a gene-environment interaction, such that certain environments allow for greater expression of genetic effects, possibly due to the availability of opportunities for individuals to show their genetic predispositions (6). While twin studies of gene-environment
