We examined four smoking phenotypes previously shown to be heritable in the African and European ancestry samples18, 19, 20, 21 and used in prior GWAS of smoking behavior.4, 5, 6 SI contrasted individuals who reported having smoked 100 cigarettes during their lifetime (ever smokers) with those who reported having smoked between 0 and 99 cigarettes during their lifetime (never smokers), consistent with the Centers for Disease Control classification.22 Among smokers, the age of SI (AOI) represented the age individuals began smoking. Some studies captured the age they first tried smoking, whereas others collected the age they began smoking regularly. As prior research suggests similar heritabilities and high genetic correlation between these phenotypes, we justified using either value in a general assessment of AOI. Similarly, for cigarettes smoked per day (CPD), some studies collected maximum CPD, whereas others collected average CPD. Longitudinal twin data suggests a high correlation between these variables over time, which supported using either value in our analyses. For studies that collected CPD as ranges, the mid-point of the interval was used as the data point; for example,
