It has long been recognised that risk of cannabis and other substance (ab)use runs in families. Studies aiming to understand the basis of familial risk include family studies, adoption studies, and twin studies. Family studies into cannabis use have shown moderate parent-offspring correlations (ranging between 0.30 and 0.59; (15–18) as well as sibling-sibling correlations (ranging between 0.39 and 0.59 (15, 19)). In a recent study, Merikangas et al. (20) found elevated risks for cannabis use disorders among siblings (odds ratio [OR] = 3.6), offspring (OR = 6.9), and spouses (OR = 4.4) of probands with cannabis use disorders. However, family studies cannot determine whether familial resemblance is due to genetic factors or environmental factors shared between family members. Adoption studies can distinguish genetic and shared environmental factors by comparing the similarity of the adopted child with its adopted parents and with its biological parents. To our knowledge, no adoption study has specifically examined cannabis use, but adoption studies into drug and alcohol use have found that abuse or dependence of adoptees is more related to abuse or dependence of their biological parents than their adoptive parents (21–26), indicating an important role for genetic factors.
