Possibly, the specific characteristics of the male partners and the quality of sexual relationships may be the “causal” component in the observed association but we were unable to explore this hypothesis fully as women did not report on the characteristics of the partners with whom they engaged in rVUS. Based on self-reported reasons for condom non-use (Table S6), in addition to substance use, partner discomfort was frequently reported. It is, thus, possible that early discordance in cannabis exposure was a consequence of, or subsequently resulted in, exposure of one member of the twin pair to a more non-conforming cohort of male peers (with increased access to drugs and characterized by other risky behaviors), which increased her likelihood of rVUS.
