The sample was drawn from two large population-based twin studies of the Virginia Twin Registry. The sampling and ascertainment procedures were described elsewhere [20,21]. In this study, we used a subset of twins of European ancestry and randomly selected one twin from each pair. All the subjects were unrelated. All individuals were assessed with basic smoking history, the Fagerstrom Tolerance Questionnaire (FTQ) [22] and withdrawal symptoms. The FTQ was an eight-item questionnaire (score range 0–11) widely used to evaluate the severity of nicotine dependence. In this study, the non-smokers (NS) were defined as those who never smoked a cigarette up to the time of the assessment. Regular smokers with low ND (Low-ND) were defined as those who smoked at least 5 cigarettes per week for five years and their FTQ scores were between 0–2 at their lifetime maximum tobacco consumption. Regular smokers with high ND (High-ND) were those who smoked for five years or more and had an FTQ score between 7–11. In order to evaluate separately the influence of OPRM1 on SI and ND, two measurements with overlapping but
