Abstract
Background: There has been growing interest in the associations between cigarette smoking and symptoms of depression. This study documents the comorbidity between depression and nicotine dependence in a birth cohort of 16-year-olds and examines the extent to which comorbidity between depression and nicotine dependence could be explained by risk factors associated with both outcomes. Methods: Data were gathered during the course of a 16-year longitudinal study of a birth cohort of 947 New Zealand children for (1) depressive disorders and nicotine dependence at age 16 years; and (2) prospectively measured risk factors including family social position, family history of criminality, parental smoking, life events, parental attachment, conduct problems, self-esteem, and affiliations with delinquent peers. Results: There was evidence of moderate to strong comorbidity between depression and nicotine dependence at age 16 years; teenagers with a depressive disorder had odds of nicotine dependence that were 4.6 times those of teenagers without depressive disorder. Analyses using logistic regression and log-linear modeling methods revealed that a substantial component of the comorbidity between depression and nicotine dependence was explained by common or correlated risk factors associated with both outcomes. After adjustment for common or correlated risk factors, the adjusted odds ratio between depression and nicotine dependence was 2.3. Conclusions: Comorbidities between depression and nicotine dependence seem to be well established by the age of 16 years. Much of this comorbidity can be explained by common or correlated risk factors associated with depression or nicotine dependence.