Background. The aim of this paper was to examine the association between major depression and cigarette smoking among young adults in a birth cohort before and after adjusting for confounding factors.

Method. Data were gathered over the course of the Christchurch Health and Development Study (CHDS). The CHDS is a longitudinal study of a birth cohort of 1265 New Zealand children studied to age 21. Data were gathered by interview on: (a) major depression over the period 16–21 years; (b) daily smoking and nicotine dependence over the period from 16–21 years. In addition, the study included extensive information on social, family, and behavioural factors in childhood and adolescence.

Results. Young people meeting DSM-IV criteria for major depression had elevated rates of daily smoking and nicotine dependence. These associations were reduced substantially by control for potential confounding child and adolescent factors. Nonetheless, even after such control, major depression was associated with increased rates of daily smoking (IRR=1·19; 95% CI=1·03, 1·39) and elevated rates of nicotine dependence (OR=1·75; 95% CI=1·13, 2·70).

Conclusions. The results suggest that much of the association between smoking and depression reflects common confounding factors that are associated with both outcomes. Nonetheless, even after control for these factors there is evidence of a possible causal linkage between smoking and depression. The direction of causality between smoking and depression remains unknown.