Abstract
INTRODUCTION: Individuals who quit smoking frequently experience symptoms of anxiety and/or depression. It is not clear whether these symptoms index liability to negative affect generally or whether such symptoms are a function of nicotine withdrawal and are thus indexed by nicotine dependence (ND). METHODS: A population-based sample of twins (N = 4,777 individuals) reported their lifetime history of psychopathology, ND, and symptoms of anxiety and depression experienced after attempts to quit smoking. Co-twin phenotype was used to predict withdrawal-induced symptoms of negative affect and to test whether genetic factors influence liability to these symptoms. RESULTS: Co-twin's ND was significantly associated with nicotine withdrawal-induced symptoms of anxiety and depression. Furthermore, monozygotic co-twins more strongly predicted outcome than did dizygotic co-twins, indicating that genetic factors contribute to risk. Co-twins' history of psychopathology did not predict outcome, suggesting that liability to withdrawal-induced negative affect is independent of a liability to negative affect outside the context of nicotine withdrawal. CONCLUSIONS: These findings indicate that symptoms of anxiety or depression experienced in the context of nicotine withdrawal are best conceptualized as a component of the withdrawal syndrome, with the severity of symptoms indexed by level of ND. Genetic influences underlying ND contribute to the liability to these symptoms. Though psychopathology might be indirectly related to withdrawal-induced symptoms through its correlation with ND, it is not directly predictive of withdrawal symptoms.