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 in risk for reaching these smoking milestones. Examples of measured environmental influences shown to contribute to smoking initiation, persistence and dependence include parent-child conflict and parent-child closeness, sibling, peer and friend smoking and substance use. Increased risk of smoking initiation and regular smoking are also associated with parenting style (Duncan et al. 1998; Huver et al. 2007). Older sibling smoking and drug use is a strong predictor of cigarette use by other offspring (Mayhew, Flay & Mott 2000). Numerous studies have implicated the strong effect of friend and perceived peer smoking (Castrucci et al. 2002) as predictors of young adult and adolescent smoking habits. However it is not known if peer smoking and other family and non-family environmental variables remain significant predictors of adolescent and young adult smoking after controlling for familial vulnerability.
