Background. Depression is associated with smoking, but the causality of the relationship is debated. The authors examine smoking behaviour as a predictor of depression among the Finnish adult twin population.

Method. Based on responses to surveys in 1975 and 1981, the authors characterized the subjects as never smokers, persistent former smokers, quitters, recurrent smokers and persistent smokers. The Beck Depression Inventory (BDI) was applied in 1990 to measure depression (BDI score >9). Although the population consisted of twins, the authors first considered the subjects as individuals. Logistic regression models were computed for 4164 men and 4934 women. In order to control for family and genetic background, conditional logistic regression analyses were conducted among twin pairs discordant for depression. Bivariate genetic modelling was used to examine genetic and environmental components of the correlation between smoking and depression.

Results. Among the men, persistent smoking (OR 1·42, 95% CI 1·07–1·89) and smoking in 1975 but quitting by 1981 (OR 1·68, 95% CI 1·17–2·42) was associated with a higher risk of depression, while among the women only the quitters had an elevated risk (OR 1·38, 96% CI 1·01–1·87). The gender×smoking interaction showed persistent smoking to be a stronger risk for men. When family and genetic background were controlled, smoking remained a predictor of depression. Genetic modelling among the men suggested a modest correlation (rg=0·25) between genetic components of smoking and depression.

Conclusions. Smoking behaviour may be a gender-sensitive predictor of depression, the stronger association in men being partly accounted for by having underlying genes in common.