Around 60% of adolescents have smoked a cigarette at some point in their lives and around a third have smoked during the last month. Tobacco is often seen as a gateway to cannabis and other drugs but some scientists dispute this 'gateway' theory and suggest that there is a common genetic factor that links tobacco smoking and cannabis use. Researchers from the University of Amsterdam studied 3,744 Finnish twins and found that the 'total variation in the initiation of illicit drugs' was down to starting smoking (40%) genetic factors (32%) common environmental factors (20%) and unique environmental factors i.e. factors experienced by only one of the twins (8%) suggesting that tobacco use was marginally more important than genetics in predicting illegal drug use.
Huizink, Anja C. ... [et al] - Tobacco, cannabis, and other illicit drug use among Finnish adolescent twins: causal relationship or correlated liabilities Journal of Studies on Alcohol and Drugs January 2010, 71(1), 5-14