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Twin and Adoption Studies

Twin studies focus on the relationship of identical twins to fraternal twins and their varying rates of alcoholism. If alcoholism is inherited, identical twins ought to become alcoholics at greater rates than fraternal twins because they share 100% of their genes — in contrast, fraternal twins share only 50% of their genes. A number of studies have consistently shown higher concordance rates for drinking behavior and possibly alcoholism in identical twins compared with fraternal twins. The heritability estimates for genetic influence account for between 50% and 60% of the variation in the risk for alcoholism in males. Studies of female twins, in contrast, however, demonstrate smaller rates of concordance, although studies using population-based twin registries have found that genetic influences on alcoholism to be of similar magnitude in both males and females.

What evidence is found from adoption studies? Adoption studies examined biological children of alcoholic parents adopted into nonalcoholic homes and biological children of nonalcoholic parents adopted into alcoholic homes. If environment plays a greater role, then the home a child is raised in will have the greatest influence in the development of alcoholism. If, on the other hand, genes play a greater role, then the child's biological parents will have the greatest influence in the development of alcoholism. In all studies of male adoptees, those whose biological parents were alcoholic were at a significantly higher risk for alcoholism than were children whose biological parents were not alcoholic (i.e., 1.6 to 3.6 times greater). These data portray a genetic contribution to the risk for developing alcoholism. The studies of female adoptees demonstrated mixed results, perhaps providing some evidence of possible sex differences in heritability, but the numbers of alcoholic female adoptees in the studies were too small to draw any definitive conclusions.

In the laboratory, one is able to measure subjective and objective responses to the consumption of alcohol and compare those responses between sons of alcoholics and nonalcoholics. The sons of alcoholics report decreased subjective ratings for feeling intoxicated, and they objectively had evidence of intoxication given the same amount of alcohol as sons of nonalcoholics. The study population consisted of white male college students who drank alcohol but were not alcohol dependent. Ten-year follow-up data were recently published for these young men. Of the sons of alcoholics, 26% were alcohol dependent by 30 years old, as opposed to 9% of the control group whose fathers were not alcoholic. Furthermore, 56% of the sons of alcoholics who reported fewer objective and subjective reactions to alcohol became alcohol dependent, as opposed to 14% of the sons of alcoholics who did not report decreased reactions. Few sons of nonalcoholic fathers became alcoholic. There are some biological propensities toward alcoholism that are independent of but may be influenced by genes. Finally, a study examining the brains of alcoholic versus nonalcoholic subjects found that the amygdala is smaller in subjects with family histories of alcoholism, suggesting that inherited differences in brain structure may also affect risk. The amygdala is an area of the brain that is thought to play a role in the emotional aspects of craving, which can lead to addiction.

Amygdala attached to the tail of the caudate structure of the brain that is considered a part of the limbic system.

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