Using the same study population, the Harvard Twin Study of Substance Abuse that was conducted by Tsuang et al aimed to determine the genetic, and environmental factors that contribute to the heritability of various classes of drugs [53]. Genetic influences (A) contributed 34% of the total variance across all drugs overall, with significant proportions of variance accounted for by additive genetic influences in marijuana (0.33), opiate (0.43), and stimulant abuse (0.44) (p<0.05) [53]. Taken together, these results suggest a moderate genetic contribution to opioid abuse and dependence.
