The goal of the current study was to test previously observed relationships between cannabis and brain volumes in a large normative sample of twins/siblings from the Human Connectome Project (HCP; N=483). First, we examined whether cannabis exposure, age of onset of use, and lifetime frequency of use predicted whole brain volume (WBV), amygdala, hippocampus, VS, or OFC volumes. Second, we quantified the degree to which shared genetic and individual-specific environmental factors contributed to these associations. Finally, to test whether any significant volumetric differences could be attributed to predispositional/familial or causal factors, we compared volumes across sex-matched twin/sibling pairs (henceforth referred to as sibling pairs) discordant for cannabis exposure (Npairs=89), concordant for exposure (Npairs=81), or concordantly unexposed (Npairs=71).
