1991). Nicotine dependence was then classified into mild (FTND score 0–3), moderate (FTND score 4–6), or severe (FTND score 7–10), as has been done in previous research (Hancock et al., 2015). Current/former smoker was coded as a dichotomous phenotype, where current smokers reported at interview that they presently smoked and former smokers had quit smoking at least 1 year before interview (Tobacco and Genetics Consortium, 2010). The phenotypes of age of onset, cigarettes per day, nicotine dependence, and current/former smokers included only ever smokers. Schizophrenia was also coded as a dichotomous phenotype based on meeting DSM-IV diagnostic criteria for schizophrenia or schizoaffective disorder (Schizophrenia Working Group of the Psychiatric Genomics Consortium, 2014).
