We used heavy and light smoking rather than nicotine dependence because of the questionnaire information collected from the study participants. The number of cigarettes smoked per day is highly heritable (25) and predictive of nicotine dependence (3, 6). To evaluate how our cigarettes per day phenotype compared to that defined by FTND scores, we compared these characteristics in another study in which over 16,000 smokers, ages 25-44, provided information on both parameters. These smokers were participants in the Collaborative Study of Nicotine Dependence (COGEND) (26). As shown in Figure 2, over 94% of the respondents who reported currently smoking 5 or fewer cigarettes per day had a lifetime maximum FTND score of less than 4 which is consistent with a classification of non-dependent, and 84% had an FTND score of 1 or less. Over 96% of current smokers who reported smoking 30 or more cigarettes per day were nicotine dependent using the FTND criteria of a score of 4 or more. Similar findings were seen in former smokers. This comparison of number of cigarettes per day and FTND score in
