Drinking behavior in the Colorado adoptee and twin sample

Using a sample of adult Colorado twins, nontwin sibling pairs and pairs of unrelated adoptees reared together (N = 346), the extent of the similarity in drinking behavior within pairs was estimated. The analyses, which go beyond those possible with a study confined just to twins, suggest that some alcohol-drinking behaviors may be genetically influenced. However, the genetic variance appears to be primarily nonadditive so that it contributes more to differences than to similarities between first-degree relatives. Alternatively, although less likely, these results might suggest a special common environment for identical twins that is not present for fraternal twins, nontwin siblings or unrelated adoptees reared together. Environmental analyses indicate that both types of twins share a special family environmental similarity that is not shared by nontwin siblings. Environmental influences that affect individual differences in drinking behavior appear to contribute more to dissimilarity than to similarity. The results do not support the contention that being raised in the same family contributes substantially to similarity in characteristics of drinking behavior.