According to the article, although considerable evidence suggests that genetics plays a role in the development of alcohol dependence (AD), the offspring-of-twins design has been used infrequently to assess these effects. This design allows researchers to look at both genetic and environmental risk factors based on the twin's history of alcoholism. For example, children raised by an alcoholic monozygotic (MZ, or identical twin) or dizygotic (DZ, non-identical twin) twin parent are at high risk for psychiatric disorders including alcoholism because they are at both high genetic and environmental risk (studies have shown that children raised by alcoholics are more likely to become alcoholics themselves). In contrast, children raised by the non-alcoholic MZ twin of an alcoholic are at a low environmental risk because the children did not grow up in an alcoholic household. However, these children have the same high genetic risk as the children raised by the alcoholic twin because the fathers have the same genes. Children raised by the non-alcoholic DZ twin of an alcoholic are also at a lower environmental risk for alcohol dependence, but are at an intermediate genetic risk because DZ twins share only half their genes.
Theodore Jacob, Ph.D., of Palo Alto Veterans Affairs Health Care System, Menlo Park, Ariz., and colleagues used the offspring-of-twins design to study the effects of genetics and environment on the risk of developing alcohol dependence.
The researchers conducted telephone interviews with 1,213 male MZ and DZ twins who had been enrolled in the Vietnam Era Twin Registry. The registry is composed of male-male twin pairs born between January 1, 1939 and December 31, 1957, who served in the U.S. military between May 1, 1965 and August 31, 1975. This study included registry members who had completed an interview in 1992 and reported having children born between 1974 and 1988. The researchers also interviewed 1,270 children of the twins and 862 mothers of these children. The interviews consisted of detailed information about psychiatric disorders, including alcohol abuse and alcohol dependence. The fathers and mothers averaged 50 and 47 years old.
This study focuses on the diagnosis of alcohol abuse or alcohol dependence among the offspring of the twins.
The offspring ranged in age from 12 to 26 years old, whereas the average age of the twins fathers was 50 years old and the average age of the mothers was 47 years old. Among offspring, 276 had MZ or DZ twin fathers who had no alcohol abuse or alcohol dependence. In the rest of the offspring, either their twin father or their twin father's brother, or both had alcohol abuse or alcohol dependence.
The researchers found that children of MZ and DZ twins with a history of alcohol dependence were significantly more likely to exhibit alcohol abuse or alcohol dependence than were children of non-alcoholic fathers. Children of an alcohol-abusing MZ twin whose co-twin was alcohol dependent were more likely to be alcohol dependent than children of non-alcoholic twins. Children of a MZ twin with no history of alcohol abuse or dependence whose co-twin was alcohol dependent were no more likely to be alcohol abusers or alcohol dependent that the children of non-alcoholic twins.
The researchers conclude: "These finding support the hypothesis that family environment effects do make a difference in accounting for offspring outcomes, in particular, that a low-risk environment (i.e., the absence of parental alcoholism) can moderate the impact of high genetic risk regarding offspring for the development of alcohol-use disorders."
(Arch Gen Psychiatry. 2003;60:1265-1272. Available post-embargo at archgenpsychiatry.com) Editor's Note: This study was supported by grants from the National Institute of Alcohol Abuse and Alcoholism and by a Merit Review Grant from the Department of Veterans Affairs Medical Research Service, Washington, D.C. (Dr. Jacob).
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Journal
Archives of General Psychiatry