From these studies we can see that the environment moderates the effect of genes across different cigarette use phenotypes. However, the extent to which the environment moderates the genetic influences across different cigarette use phenotypes varies. There are a couple of different reasons why it is the case that GxE is found for some cigarette use phenotypes and not others. It could be that the genes influencing initiation may be different from the genes influencing other cigarette use behaviors (96,107,108), such as adolescent smoking, daily smoking, number of cigarettes smoked per day, nicotine dependence, and smoking cessation. It is also possible that, the effect of certain environmental measures of smoking that are potentially influenced by both genes and environment [e.g. smoking status of father, sibling, friend, or best friend (35)] seems to vary among carriers of nicotinic receptor genes, but not among carriers of dopaminergic gene variants - possibly, suggesting that either: the effect of nicotinic receptor genes is larger than that of dopaminergic genes or that the effect of dopaminergic genes does not vary as a function of environmental
