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
