A bivariate twin study of cannabis use and regular tobacco smoking across three different countries

Background: Regular cigarette smoking and cannabis use are strongly related to each other, yet few studies explore their underlying variation and covariation. We evaluated the genetic and environmental decomposition of variance and covariance of these two traits in twin data from three countries with different social norms and legislation. Methods: Data from the Netherlands Twin Register, FinnTwin12/16, and the Minnesota Center for Twin Family Research were analyzed in bivariate threshold models of lifetime smoking and cannabis use (N=21,617). We ran unstratified models and models stratified by sex and country. Results: Prevalence of lifetime smoking was lowest in the Netherlands and prevalence of lifetime cannabis use was highest in Minnesota. In the unstratified model, genetic (A) and common environmental factors (C) contributed substantially to the liabilities of lifetime smoking (A=0.47, C=0.34) and lifetime cannabis use (A=0.28, C=0.51). The two liabilities were significantly phenotypically (rP=0.56), genetically (rA=0.74), and environmentally correlated in the unstratified model (rC=0.47and rE=0.48, representing correlations between common and unique environmental factors). The magnitude of phenotypic correlation between liabilities varied by country but not sex (Minnesota rP~.70, Netherlands rP~0.59, Finland rP~0.45). Comparisons of decomposed correlations could not be reliably tested in the stratified models. Discussion: The prevalence and association of lifetime smoking and lifetime cannabis use vary by sex and country. These two behaviors are correlated because there is genetic and environmental overlap between their underlying latent liabilities. There is heterogeneity in the genetic architecture of these traits across country.