In this study, we used a subset of twins of European ancestry and randomly selected 1 twin from each pair. All the study participants were unrelated. All individuals were assessed with basic smoking history and the Fagerström Tolerance Questionnaire (FTQ)44 or the Fagerström Test for Nicotine Dependence (FTND).45 The FTQ was an 8-item questionnaire (score range, 0-11), and the FTND was a 6-item questionnaire (score range, 0-10). Both the FTQ and the FTND have been widely used to evaluate ND. The first sample, denoted as the Virginia Study of Nicotine Dependence (VAND), contains 688 individuals. For this sample, we used a 3-group design: nonsmokers (n=244, 164 men and 80 women), defined as those who never smoked a cigarette up to the time of the assessment; regular smokers with low ND (n=215, 151 men and 64 women), defined as those who smoked at least 5 cigarettes per week for 5 years and had FTQ scores between 0 and 2 at their lifetime maximum tobacco consumption; and regular smokers with high ND (n=229, 150 men and 79 women), defined as those who smoked for 5 years or more and had an FTQ score between 7 and 11. This 3-group design was to
