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Precursors of addiction
Genetic and environmental influences on adolescent drug use
Edward R. Winstead

Saying no to drugs during adolescence lessens the likelihood of addiction later in life. Conversely, starting drug use in adolescence is a predictor of drug use in adulthood. Relatively few studies have investigated the environmental and genetic risk factors involved in teenager drug use compared to those in adults. This month, researchers report new findings from a longitudinal study tracking drug use and abuse in a population of Minnesota adolescents.

Researchers at the University of Minnesota analyzed data on the use of legal (tobacco) and illegal substances (marijuana and amphetamines) in a sample of 17-year-old twins. The researchers found that behavioral differences among the group were more likely to be due to environmental factors than to genetic factors. By statistical measures, genes were a significant factor in tobacco use and nicotine dependence but not in the use of illegal drugs. This statistical difference may reflect the relative difficulty of obtaining illegal versus legal substances.

Matt McGue, of the Department of Psychology at the University of Minnesota, led the research. McGue and colleagues write in their paper: "The most striking finding in our study was that shared environmental factors (e.g., sharing a household where drugs are available through parents or siblings or sharing the same circle of acquaintances) seem to exert a considerable influence on illicit drug use and abuse in adolescence." The paper appeared in a recent issue of the American Journal of Medical Genetics.

The finding of modest genetic and strong environmental influences on substance use in this population may not be the whole story. Research on substance abuse in adults suggests that genetic influences may become stronger over time. "We expect that some of the adolescents we classified as 'unaffected' at age 17 will ultimately be affected by a substance disorder once they pass completely through the period of greatest risk," the authors write. Future evaluations of the data may "substantially change the relative contributions of genetic and environmental factors that we have reported here." (Preliminary data on twins who are now 20 years old is consistent with this expectation, according to the paper.)

The researchers hypothesized that sex differences might be a factor in the results. In general, there were no significant differences between the sexes or between fraternal and identical twins. "There was no evidence that gender moderated the strength of genetic influences," the authors write. The sample included 289 sets of male twins and 337 sets of female twins.

The current data are consistent with findings published last year by the Minnesota researchers using a subset of this population of twins. The finding of significant environmental influences on teenage drug use, the researchers concluded, "underscores the importance of intervention on early adolescent substance use."

The data are also consistent with those published last year by researchers at Virginia Commonwealth University. The Virginia team analyzed data on a younger population (twins aged 8 to 16). The study found that genetic factors explained a significant proportion of the variation in the use of tobacco, alcohol, and other drugs. In the Journal of Studies on Alcohol, the researchers wrote: "Shared environmental factors contributed significantly to lifetime alcohol use and other drug use."

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McGue, M., Elkins, I. & Iacono, W.G. Genetic and environmental influences on adolescent substance use and abuse. Am J Med Genet (Neuropsychiatr Genet) 96, 671-677 (October 9, 2000).
Han, C., McGue, M.K. & Iacono, W.G. Lifetime tobacco, alcohol and other substance use in adolescent Minnesota twins: univariate and multivariate behavioral genetic analyses. Addiction 94, 981-993 (July 1999).
Maes, H.H. et al. Tobacco, alcohol and drug use in eight- to sixteen-year-old twins: The Virginia twin study of adolescent behavioral development. J Stud Alcohol 60, 293-305 (May 1999).

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