In this study, we used 2 independent samples of white patients aged 18 to 65 years, both drawn from 2 large population-based twin studies of the Mid-Atlantic Twin Registry. The sampling and ascertainment procedures for this study have been described elsewhere.5,42,43 Briefly, female-female twin pairs born between 1934 and 1974 became eligible if both members responded to a mailed questionnaire in 1987-1988. Data on smoking history and ND used in this report were collected at the fourth wave of interviews conducted in 1995-1997. Data on the male-male pairs born between 1940 and 1974 were collected at the second wave of interviews conducted in 1994-1998. The mean (SD) age and educational level of the twins were 36.3 (8.2) years and 14.3 (2.2) years, respectively, for the female-female pairs and 37.0 (9.1) years and 13.6 (2.6) years, respectively, for the male-male pairs. 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 evaluate separately the influence of CNR1 on SI and ND, 2 measurements with overlapping but not identical genetic effects.5,46 To estimate the influence of CNR1 on SI, we compared the allele frequencies of testing SNPs between the nonsmokers and regular smokers (which included both the low- and high-ND groups). To estimate the influence of CNR1 on ND, we compared the allele frequencies between the low- and high-ND groups. Smokers with FTQ scores between 2 and 7 were not used in this dichotomized design. The second sample, denoted as the Virginia Study of Anxiety and Neuroticism (VAANX), was a sample initially selected for the study of anxiety and neuroticism. We used the software package Mx (http://www.vcu.edu/mx/) to perform a multivariate genetic analysis to identify a latent phenotype that reflected genetic covariation (ie, shared genetic susceptibility) across the 6 phenotypes (major depressive disorder, generalized anxiety disorder, panic disorder, agoraphobia, social phobia, and neuroticism [Hettema et al47 provide the details]). The inclusion criterion was the top and bottom 25th percentile of a genetic factor score—a composite index that represented several internalizing anxiety and neuroticism phenotypes.48 The cases had a mean raw neuroticism score of 6.30 (z score = 1.04) and had the following frequencies of the target psychiatric illnesses: major depressive disorder (80.1%), generalized anxiety disorder (53.8%), panic disorder (20.5%), agoraphobia (14.1%), and social phobia (17.5%). The controls were free of the 5 disorders and had a mean raw neuroticism score of 0.55 (z score = −0.89). This sample contains 1128 individuals, of whom 6 were included in the VAND sample and another 161 were co-twins of the participants in the VAND. To maintain the independence of the 2 samples, these 167 overlapping individuals were removed from all analyses conducted with the VAANX sample. Because the VAANX was selected by the genetic factor score, there were not enough individuals with high and low FTQ or FTND scores for a dichotomized design, so we used a quantitative design (FTND scores) to assess ND in this sample. As defined in the VAND sample, individuals who reported never having smoked regularly and whom we, therefore, did not attempt to assess with the FTQ or FTND were classified as nonsmokers. All others were classified as smokers, including those who were not smoking at assessment but had smoked previously. The remaining 961 individuals included 532 nonsmokers (299 men and 233 women) and 429 smokers (276 men and 153 women). The distributions of FTND scores for these individuals are shown in Figure 1. For the study participants, both the FTQ and FTND scores were negatively correlated with the level of education (polychoric correlation: FTQ, r = −0.31 [P = .04]; FTND, r = −0.33 [P = .04]) and socioeconomic status as measured by yearly income (polychoric correlation: FTQ, r = −0.20 [P = .047]; FTND, r = −0.23 [P = .05]).