To learn more about how genetics and environment feed the development of alcohol dependence, researchers at Harvard and Boston University did a good old-fashioned study of twins.

What the researchers wanted to know: What role do genetics and other factors play in the age at which someone starts drinking and the age at which he becomes dependent?

What they did: This is an unusual group of subjects: the Vietnam Era Twin (VET) Registry consists of 7,375 pairs of male twins who served in the U.S. military during the Vietnam War. For this study, the researchers used the 3,372 pairs for which it was known whether they were identical or fraternal. (About 55 percent were identical.) In 1992, the brothers were interviewed over the phone—including questions about alcohol use, like how often they use it, when they started, and how it affects their mood. Twin studies like this are a way to tell whether some condition happens because of genetics or environment (since there are both identical and fraternal twins).

What they found: Genetic factors influenced when the men started drinking regularly and how fast they moved from drinking to being diagnosed with alcohol dependence.

What the study means to you: Yes, some amount of alcoholism is because of your genes.

Caveats: There were no women in the study, the men were ages 33 to 52, and most were Vietnam vets. Also, all twin studies assume that identical twins' and fraternal twins' environments are equally similar.

Find out more: A detailed medical explanation of alcoholism from MedlinePlus

Guess what? There's an International Society for Twin Studies.

Read the article: Liu, I-C. et al. "Genetic and Environmental Contributions to the Development of Alcohol Dependence in Male Twins." Archives of General Psychiatry. September 2004, Vol. 61, pp. 897–903.

Abstract online: http://archpsyc.ama-assn.org