that experiences of childhood maltreatment cluster within pairs of identical and fraternal twins that are raised together (Schulz-Heik et al., 2010; Young-Wolff et al., 2011), although the extent to which these familial factors are environmental or genetic has not been explored. Familial factors, both genetic and environmental, play a prominent role in the etiology of cannabis use, while familial environmental factors appear to be less influential for cannabis-related problems. In Verweij et al.’s meta-analysis of twin studies of cannabis use and problem use (2010), estimates of genetic influences on the two outcomes were 40–48% and 51–59%, respectively, whereas estimates of shared environmental influences were 25–39% for initiation but only 15–20% for problem use. Nonetheless, the extent to which associations between cannabis use, CUD and childhood maltreatment can be attributed to common familial factors, either genetic or shared environmental, and to individual-specific experiences remains unknown.
