Detailed information on the evolution of smoking behavior as well as relevant covariates was retrospectively collected through structured telephone interviews, as previously described (Loukola et al. 2014). Questions used to inquire the several milestones in smoking behavior are presented in Table 1. We constructed four time‐to‐event phenotypic variables based on the transitions measured in years between progressive smoking states: (1) smoking initiation (years from birth to the age of smoking the first cigarette), (2) persistent smoking (years from the age of smoking the first cigarette to the age of daily smoking), (3) tolerance (years from the age of daily smoking to the age when the heaviest smoking started), and (4) cessation (years from the age of daily smoking to the age of successful quitting). For smoking cessation, we defined continuous abstinence of more than 6 months as successful quitting, as previously suggested (Hughes et al. 2003), and those individuals still smoking by the time of the interview were treated as censored. We excluded subjects (N = 70) abstinent for less than 6 months prior to the interview because their cessation
