In the first major twin study to compare genetic and environmental factors that contribute to the risk of alcoholism in both sexes, researchers have found that genetics plays an important role in determining alcohol dependence in women as well as in men. The study contradicts the long-held assumption that a woman's environment is more likely to influence whether she becomes dependent on alcohol.
The study was conducted by scientists at Washington University School of Medicine in St. Louis and Australian collaborators at the Queensland Institute of Medical Research. It was funded by the U.S. National Institute of Alcoholism and Alcohol Abuse, and the findings were published in a recent issue of the journal Psychological Medicine.
While there is ample evidence for an important genetic influence on alcoholism risk in men, the tie between genetics and alcoholism in women has been uncertain, said the paper's lead author, Andrew Heath, Ph.D., a professor of psychiatry at Washington University.
The study sought to address this shortcoming. It involved 2,685 pairs of twins, all participants in an adult twin study started in Australia in 1978 and maintained by the Australian National Health and Medical Research Council. The two members of each pair were raised in the same home environment. The study included -- for the first time -- twins of opposite sex as well as twins that were either both male or both female.
Telephone interviews with the twins were conducted to assess whether the participants had lost control over their drinking, were unable to cut back on drinking or had similar problems with alcohol.
Twins who had an alcoholic identical twin were much more likely to be alcoholic themselves than were twins who had an alcoholic fraternal twin, and this was equally true in women and in men. More surprisingly, men who had an alcoholic twin sister had very high rates of alcoholism.
"If shared environmental factors are of predominant importance in women but genetic influences predominate in men -- a commonly held belief -- then we would have expected to see very low concordance for alcoholism of unlike-sex twin pairs," Heath said. "That wasn't the case."
The study also found that women with an alcoholic identical (monozygotic) twin sister were six times more likely to be alcohol dependent than other women. Monozygotic twins share the same genetic makeup. Dizygotic, or nonidentical, twins, who are not more alike genetically than ordinary sisters, were only three times more likely to be alcohol dependent if their twin had a drinking problem.
The study also examined whether psychiatric disorders that commonly precede or coexist with alcoholism, such as childhood behavior problems like lying and stealing or depression, were as strongly associated with risk of alcoholism in women as in men.
"It used to be believed that depression predicted increased risk of alcohol problems in women, whereas a history of childhood behavior problems predicted alcoholism risk in men. We found that depression is a potent predictor for alcohol dependence in both men and women," said Heath. "A history of behavior problems also is as strong a predictor in women than in men -- slightly more powerful in women, in fact."
The researchers acknowledge there still is much to learn about how and why alcoholism manifests itself. What they did learn, however, was that nearly two-thirds of the differences in risk of alcoholism among adults in the general population could be attributed to underlying genetic differences.
"As a point of comparison, say we were talking about lung cancer," Heath explained. "We'd expect to find that a large proportion of the differences in the general population's risk for lung cancer was explained by differences in smoking habits. For alcoholics, male and female, we're saying that a major portion of the difference in risk can be attributed to underlying genetic differences."
Heath hopes the study will serve as a warning sign for women who know they have alcoholic relatives. "Our hope is that our research will refocus attention on alcohol problems in women, particularly women at genetic risk with relatively mild alcohol problems that often go undetected by families and physicians," he said. "Often, these people don't get treatment -- until it is too late. Despite what people used to believe about differences in the causes of alcoholism in women and men, it is the similarities rather than the differences that are most striking. As increasing numbers of young women drink heavily, rates of alcoholism in women are likely to increase."
Note: For more information, refer to Heath AC, Bucholz KK, Madden PAF, Dinwiddie SH, Slutske WS, Bierut LJ, Statham DJ, Dunne MP, Whitfield JB, Martin NG. "Genetic and Environmental Contributions to Alcohol Dependence Risk in a National Twin Sample -- Consistency of Findings in Women and Men". Psychological Medicine. 27(6):1381-1396, 1997 November.
Journal
Psychological Medicine