It is well established that genetic factors play an important role in smoking milestones from initiation to nicotine dependence (ND). Genes, shared and unique environmental factors contribute to smoking initiation (ever trying cigarettes) and regular smoking (i.e. self report of ever being a smoker, ever smoking 100 cigarettes) (Madden et al. 2006; True et al. 1997;), while persistent smoking and ND appear to be due to genetic and unique environmental factors (Madden et al. 2006; True et al. 1997; True et al. 1999). The estimates of the genetic contribution to regular smoking range between 58–74% (Heath & Martin 1993; Madden et al. 2006; Pergadia et al. 2006a; Pergadia et al. 2006b; True et al. 1997; True et al. 1999). Genetic factors have been found to account for 33–70% of the variance in risk for developing ND (Heath & Madden 1995; Heath & Martin 1993; Kendler et al. 1999; Lessov et al. 2004; Maes et al. 2004). Genetically-informative studies have not included measured environmental influences in models to determine the nature of the environmental factors that account for the remaining variance
