Overall, our meta-analyses, by aggregating the results of a number of previous twin studies, provided more robust estimates of the genetic and environmental influences on cannabis initiation and problematic use. Because our analyses average estimates over samples of different sizes and demographic makeup, our findings are likely to be more generalisable than the source studies. Our results indicate that A, C and E factors each contribute significantly to vulnerability to both cannabis use initiation and problematic use. This confirms that cannabis problems do not have a single or simple cause, and suggests that both genetic and environmental factors are potential targets for treatment and prevention measures.
