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The Harvard Twin Study of Substance Abuse: What We Have Learned

Tsuang, Ming T. MD, PhD; Bar, Jessica L. MA; Harley, Rebecca M. PhD; Lyons, Michael J. PhD

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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 8000 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 unique environmental factors on the abuse of illicit substances. Multivariate analyses have indicated that the co-occurrence of abuse of various types of illicit drugs reflects a common vulnerability, influenced by both genetic and environmental factors, that cuts across all categories of illicit drugs. We have also demonstrated that some drugs have unique determinants, both 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 transitions 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 genetic influences shared by both substances. We found that the co-occurrence of illicit drug abuse and major depression is due to unique environmental influences. The phenotypic association between symptoms of conduct disorder and alcohol and marijuana dependence is due largely to shared environmental influences. Our results, thus far, indicate a complex pattern of genetic and environmental influences on substance use and abuse.

From the Harvard Institute of Psychiatric Epidemiology and Genetics (Drs. Tsuang and Lyons), Massachusetts Mental Health Center (Dr. Tsuang), and the Department of Psychology, Boston University (Ms. Bar, Drs. Harley and Lyons), Boston, Mass.

Supported by a grant (DA04604) from the National Institute on Drug Abuse (Dr. Tsuang), and the Department of Veterans Affairs Health Services Research and Development Service and Cooperative Studies Program (Study 992).

Original manuscript received 20 March 2001; revised manuscript received 20 June 2001, accepted for publication 25 June 2001.

Reprint requests: Ming T. Tsuang, MD, PhD, Massachusetts Mental Health Center, 74 Fenwood Rd., Boston, MA 02115 (e-mail: ming_tsuang@hms.harvard.edu).

© 2001 President and Fellows of Harvard College

© Copyright Oxford University Press 2001.
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