Cigarette smoking and depression: tests of causal linkages using a longitudinal birth cohort

BACKGROUND: Research on the comorbidity between cigarette smoking and major depression has not elucidated the pathways by which smoking is associated with depression. AIMS: To examine the causal relationships between smoking and depression via fixed-effects regression and structural equation modelling. METHOD: Data were gathered on nicotine-dependence symptoms and depressive symptoms in early adulthood using a birth cohort of over 1000 individuals. RESULTS: Adjustment for confounding factors revealed persistent significant (P<0.05) associations between nicotine-dependence symptoms and depressive symptoms. Structural equation modelling suggested that the best-fitting causal model was one in which nicotine dependence led to increased risk of depression. The findings suggest that the comorbidity between smoking and depression arises from two routes; the first involving common or correlated risk factors and the second a direct path in which smoking increases the risk of depression. CONCLUSIONS: This evidence is consistent with the conclusion that there is a cause and effect relationship between smoking and depression in which cigarette smoking increases the risk of symptoms of depression.