Analyses were conducted on two samples: 1) the full sample and 2) a subsample of participants selected for current smoking. In the subsample we selected, for each age, only those individuals who smoked at that age. All non-smoker’s CPD values were set to missing, according to the rationale that we do not know how many cigarettes they would smoke, were they in fact a smoker. We selected the subsample of current smokers because the SNPs used in this study were identified by meta-analysis for their association with CPD in current smokers. Since we are developmentally extending the meta-analytic findings, we hewed to their ascertainment method as closely as possible.1 A participant was deemed a smoker if they smoked at least once per month for the 12 months prior to assessment. This threshold was used to maximize the sample size while only keeping those individuals who were currently smoking to some extent. Stricter thresholds, such as averaging 1 cigarette per day, changed the sample size only by 15–50 people, depending on the age of assessment, and had no impact on the
