The action of these contributors, genetic or environmental, may be classified as being of a predisposing nature or person-specific. For instance, heightened corticostriatal responsivity may predate both early onset of cannabis use and risky sexual behavior (i.e. predisposing factor), or alternatively, early exposure to cannabis may result in altered ventral striatal reactivity which subsequently increases the likelihood of risky sexual behaviors (i.e. person-specific). Longitudinal data that span the developmental periods prior to onset of both cannabis use and rVUS and extend beyond their peak period of initiation are required to carefully address causal hypotheses. However, twin pairs discordant for early exposure to cannabis offer a unique opportunity for disarticulating person-specific influences (possibly, of a causal nature) from predisposing factors typically shared by members of twin pairs. In particular, for monozygotic (MZ) twins discordant for adolescent cannabis use, any excess likelihood 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
