31 or more cigarettes. Cigarettes per day was dichotomized into ≤20 (n=367, 74%) and >20 cigarettes (n=131, 26%) in the analyses as done in previous studies (Belsky et al., 2013; Broms et al., 2006). A total Fagerström Test for Nicotine Dependence (FTND) score during the heaviest period of smoking was calculated at each interview using responses to these 2 questions as well as responses to questions assessing the four remaining criteria (Heatherton et al., 1991). For the analyses, nicotine dependence was defined as a FTND score of 4 or more (n=306, 61%), which is a sensitive and specific cut-off for smoking biomarkers (Huang et al., 2008) and has been used in previous genetic studies (Bierut et al., 2007; Saccone et al., 2009).
