As an initial attempt to examine the extent to which cannabis-related phenotypes are influenced by heritable factors a series of univariate genetic models were fitted to the data on: (a) early (≤15 years) opportunity to use cannabis; (b) early onset (≤16 years) use; (c) lifetime use of cannabis on 11 or more occasions, and (d) meeting DSM criteria for a lifetime diagnosis of cannabis abuse or dependence. For each of these phenotypes, the best-fitting model was one which assumed no sex differences in the magnitude of genetic or environmental effects and the results of these analyses are summarized in Table 7. It can be seen that there were substantial heritable influences on each of the cannabis-related phenotypes with the magnitude of these influences ranging from 72% (for both early onset opportunity to use and DSM abuse/dependence) to 80% (for early onset use). The best-fitting model was an AE model and there was no significant evidence for shared environmental influences on any of these measures.
