In this study, we selected two independent samples taken from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorder [Kendler et al., [1999]; Prescott and Kendler, [1999]; Kendler and Prescott, [2007]]. These twin studies ascertained a variety of phenotypes, including substance use/abuse/dependence, neuroticism, and other common psychiatric disorders. Tobacco smoking and nicotine dependence were assessed by the Fagerström Tolerance Questionnaire (FTQ) and/or Fagerström Test of Nicotine Dependence (FTND) during the time of heaviest lifetime nicotine use [Fagerström and Schneider, [1989]; Heatherton et al., [1991]]. The VAANX-ND sample was a combined sample of two previously used panels, the Virginia study of nicotine dependence (VAND) panel and the Virginia study of anxiety and neuroticism (VAANX) panel [Chen et al., [2004], [2008]; Hettema et al., [2006]]. The main reason to combine these two panels together was to increase sample size and statistical power. Because the VAND panel was originally designed to study both smoking initiation and nicotine dependence [Sullivan et al., [2001]; Chen et al., [2004], [2007]], never smokers were included. There were two groups of smokers in the VAND panel,
