Many of our observations are consistent with other large cohort studies. Increasing age is associated with increasing smoking and ND (Hu, Davies & Kandel 2004) in adolescents and young adults. And our finding for an association between white race and regular smoking is consistent with previous studies in the United States (Park, Weaver & Romer 2009). Previous studies have demonstrated peer smoking is significantly associated ever smoking (Hu, Davies & Kandel 2004), transitions to daily smoking (Bricker et al. 2007) and with developing ND (Hu, Davies & Kandel 2004; Kandel et al. 2007). Our study is unique in demonstrating the significance of peer influences after accounting for familial vulnerability for ND and measured environmental influences. Similarly unique is the finding that sibling substance use remained significantly associated with all offspring outcomes. This is consistent with other population based cohorts (Boyle et al. 2001) but we believe our analysis is the first to demonstrate a contribution from sibling smoking even after accounting for degree of genetic and environmental risk for ND from the parent generation.
