Here we carried out a meta-analysis of existing twin studies in order to provide a more accurate estimate of the magnitude of genetic and environmental influences on cannabis use initiation and problematic cannabis use ((symptoms of) abuse/dependence). Because cannabis use in general is more prevalent among males than females (2), and some twin studies reported sex differences in contributions of genetic and environmental factors (e.g., 33, 34), meta-analyses were carried out separate for males and females, in order to check for sex-differences in cannabis use etiology.
