A twin study of the relationship between tobacco dependence and major depression

Depression and smoking have frequently been observed to co-occur in the same individuals. We investigated the relationship between tobacco dependence (TD) and major depression (MD) using structured diagnostic interview data collected from 3,372 pairs of male veteran twins in the Vietnam Era Twin Registry. Diagnoses of major depression and tobacco dependence were based on meeting DSM-III-R criteria. We observed a strong phenotypic association between major depression and tobacco dependence (Odds Ratio {OR} = 2.8; P<.001). We then divided the subjects into four mutually exclusive categories (TD-negative/MD-negative, TD-positive/MD-negative, TD-negative/MD-positive, TD-positive/ MD-positive). "Pure" MD in one twin was associated with a significantly elevated risk of "pure" MD in the cotwin (OR = 5.9; P<.001) and "pure" TD in one twin was associated with a significantly elevated risk of "pure" TD in the cotwin (OR = 2.9; P<.001). However, "pure" MD in one twin was not associated with "pure" TD in the cotwin (OR = 0.7; P = .18) and "pure" TD in one twin was not associated with "pure" MD in the cotwin (OR= 1.1; P = .66). The phenotypic association between MD and TD does not seem to reflect a familial vulnerability influencing both outcomes.