of rVUS in the twin using cannabis at an early age, relative to her genetically identical co-twin who also shares a preponderance of her early familial environmental exposures, can be attributed to factors unique to adolescent cannabis use that may have causal characteristics. In contrast, no excess of rVUS in the twin who uses cannabis at an early age, relative to her non-using/later-onset user co-twin, would largely discount the possibility of there being any causal relationship between adolescent cannabis use and rVUS.
