Twin and family studies have shown that both genetic and environmental factors (both shared by, and specific to, family members) have an important role in the initiation of cannabis use (Kendler and Prescott 1998; van den Bree et al. 1998; Vink et al. 2010). A meta-analysis of twin studies (Verweij et al. 2010) showed that additive genetic factors explain nearly half the variance in liability to initiate cannabis use (i.e., 48 and 40 % of the variance, in females and males, respectively), while the remaining variance is accounted for—almost equally—by shared and unshared environmental factors (both about 30 %).
