The Harvard Twin Study of Substance Abuse: What we have learned

The Harvard Twin Study of Substance Abuse was carried out with the members of the Vietnam Era Twin (VET) Registry. The VET Registry comprises over 800 0 male twins who served in the United States military between 1965 and 1975 and were subsequently interviewed regarding their use of licit and illicit substances, as well as various types of psychopathology. Our research has demonstrated significant influences by genetic, shared environmental, and u nique environmental factors on the abuse of illicit substances. Multivariat e analyses have indicated that the co-occurrence of abuse of various types of illicit drugs reflects a common vulnerability, influenced by both geneti c and environmental factors, that cuts across all categories of illicit dru gs. We have also demonstrated that some drugs have unique determinants, bot h genetic and environmental, that are not shared with other drugs. In part, the genetic influence on marijuana abuse is mediated by genetic influence on subjective effects in response to the drug. The determinants of transiti ons from one stage of drug use to another differ depending on which drug or which transition is examined. We determined significant genetic influences on several aspects of nicotine and alcohol use separately, as well as gene tic influences shared by both substances. We found that the co-occurrence o f illicit drug abuse and major depression is due to unique environmental in fluences. The phenotypic association between symptoms of conduct disorder a nd alcohol and marijuana dependence is due largely to shared environmental influences. Our results, thus far, indicate a complex pattern of genetic an d environmental influences on substance use and abuse.