Results of twin studies investigating the extent to which cannabis use vulnerability is due to genetic and environmental influences have been inconsistent. We carried out the first meta-analysis of twin studies into cannabis use, in order to obtain more accurate estimates of the relative magnitude of genetic and environmental influences on cannabis use initiation and problematic use. Results for cannabis use initiation showed significant A, C, and E influences accounting for 48%, 25% and 27% of the variance in males, and 40%, 39% and 21% of the variance in females. The corresponding A, C, and E estimates for problematic cannabis use were 51%, 20% and 29% for males and 59%, 15% and 26% for females, all of which were significant. Confidence intervals for these estimates were considerably narrower than those in the source studies.
