Chunk #28 — Results — Prevalence of lifetime cannabis use, withdrawal, and abuse/dependence — Genetic analyses — The genetic and environmental influences on cannabis withdrawal
Results of the univariate ACE models for cannabis withdrawal are presented in Table 3. Given that the DOS twin pair correlation is lower (although not significantly) than the DZ same sex correlations we first fitted a general sex-limitation model, which allows for qualitative and quantitative differences in the sources of variation in cannabis withdrawal between sexes (see Neale and Cardon, 1992). To model qualitative differences in genetic influences between males and females, the genetic correlation for DOS twins was freely estimated in the model instead of fixed at 0.5 as it is for same sex DZ twin pairs. In the same way, to model qualitative differences in shared environmental influences between sexes, the C correlation for DOS twins was freely estimated in the model instead of fixed at 1.0. Under this latter model (which fitted slightly better than the genetic sex-limited model; difference in AIC=0.30), the A, C, and E estimates for males were 9%, 44%, and 47%, respectively, and for females 38%, 36%, and 26%, respectively. However, fixing the genetic or shared environmental correlation at 0.5 or 1 respectively
