Season of birth and smoking: findings from the Northern Finland 1966 Birth Cohort.

A season of birth effect in addictive disorders has scarcely been studied. As smoking is known to be a highly addictive behavior, we examined whether there exists an association between season of birth and smoking habits among the general population in the Northern Finland 1966 Birth Cohort (NFBC, n = 8,319). The birth month of each cohort member was categorized into one of the four seasons: spring (March-May), summer (June-August), autumn (September-November), or winter (December-February). Smoking habits of the cohort members were assessed by a postal questionnaire at the age of 31 yrs. Those who reported that they regularly smoked 11 or more cigarettes/day were regarded as heavy smokers. The association between season of birth and smoking was assessed with a logistic regression analysis: first, after controlling for early pregnancy-related and perinatal characteristics (Model 1) and second, after controlling for cohort members' hospital-treated psychiatric disorders, suicide attempts, adult educational level, and marital status (Model 2). Compared to males born in winter, the likelihood for heavy daily smoking was significantly increased, up to 1.3-fold, among males born in the autumn in both logistic regression models. However, among females the likelihood for heavy smoking was statistically significantly elevated among those born during any season other than winter. Season of birth may modify the development of dopaminergic or other neurotransmitter systems divergently among males and females. Altered expression of dopaminergic genes due to environmental climatic factors could explain the association between season of birth and heavy smoking.