Our latent class analysis results were consistent with findings of Madden et al. (1997) in an older cohort of female twins and Xian et al. (2005) in middle-aged male veteran twins. Like them, we found that nicotine withdrawal symptoms were best described as being arrayed on a severity continuum: 3 levels in adult smokers (no significant differences across men and women) and 2 levels in adolescent smokers (girls and boys differing only with respect to higher rates of depression and appetite). Similarly, we found that difficulty quitting, nicotine dependence (as defined by the HSI), impairment due to withdrawal symptoms, heavy smoking, depression, social anxiety, conduct disorder, and alcohol dependence were associated with increasing levels of nicotine withdrawal severity. To our knowledge, this is the first study to replicate this pattern of nicotine withdrawal, associated smoking measures and comorbidity in both adult and adolescent cohorts, in addition to examining age and gender differences. While previous studies have found ADHD to be associated with early stages of smoking, such as regular smoking (e.g., Kollins et al., 2005) in this study we found
