A few studies have specifically interrogated the genetic overlap between cannabis and tobacco involvement. In a study of U.S. twins, (34), the high correlation between cannabis and tobacco use during adolescence was primarily attributable to shared environmental factors (e.g. at age 15, RG=0 and RSE=0.75, where RG is genetic correlation and RSE is shared environmental correlation) with the role of overlapping genetic influences becoming more prominent during late adolescence and early adulthood (e.g. at age 17-19, RG=0.75 and RSE=0.25). This high degree of genetic overlap has also been found between cannabis and regular tobacco smoking (RG=0.60-0.75) (35) and between early cannabis use and nicotine dependence (RG=0.4-0.5)(36).
