Educational attainment, smoking initiation and lifetime nicotine dependence among male Vietnam-era twins

Background. Smoking initiation and persistence are clearly associated with factors commonly thought to be environmental in origin, including socio-economic status. However, twin models that incorporate gene-environment correlation and gene x environment interaction have not been applied to elucidate the genetic or environmental role that socio-economic status plays in smoking initiation and nicotine dependence. Method. Twin structural equation modelling was used to examine gene-environment correlation and gene x environment interaction of one index of socio-economic status, educational attainment, with smoking initiation and nicotine dependence among 5119 monozygotic and 4295 dizygotic male-male Vietnam-era twins from the Vietnam Era Twin Registry, a national registry of twin pairs who served in the military during the Vietnam era. Results. Educational attainment correlated significantly with smoking initiation (r=-0.27, p<0.001). Additive genetic (p=0.011), shared environment (p=0.002) and unique environment (p=0.027) components contributed to the correlation between educational attainment and smoking initiation. Educational attainment also significantly moderated the variance in smoking initiation (p<0.001), suggestive of genexenvironment interaction, although the interaction with the additive genetic, shared environmental and unique environmental components could not be resolved due to multicollinearity. In contrast, educational attainment neither correlated with nor moderated nicotine dependence, once smokers had initiated. Conclusions. Our study suggests that educational attainment is associated with smoking initiation, in part due to gene-environment correlation and genexenvironment interaction. However, once smoking initiation is taken into account, there are no effects - be they gene-environment correlation or genexenvironmental interaction - of educational attainment on nicotine dependence.