at various levels and frequencies. More recently, results from a 1981 survey of alcohol consumption patterns in the Australian Twin Register 11 , 12 revealed that adult alcohol consumption patterns are determined by separate Abstinence, Quantity, and Frequency dimensions. Moreover, for the latter two dimensions there were substantial genetic effects, whereas twin correlations for Abstinence were consistent with nongenetic determination of this dimension. What has yet to be ascertained is whether there exist low levels of alcohol or tobacco use that are the equivalent of total abstinence, and that are environmentally determined. To address such questions, large cohorts of twins with normal ranges of consumption are needed. The NAS-NRC Twin Registry has these unique features.

Finally, this study has several limitations that the reader should bear in mind. First, this is a heritability analysis of substance use, not of addiction. Second, our analysis included only male twins who were somewhat older than samples on which previous heritability analyses have been conducted. It would be interesting to see whether the effects of adjustment generalize to younger males and to females. Most important, it is, at this point, equally plausible to suggest that differences in smoking, alcohol use, or coffee consumption behavior cause or result from twin similarity or dissimilarity on the variables used in this analysis. Several aspects of the data also limit inferences regarding the differential impact of contact between the twins on twin similarities of appetitive behaviors. For example, although we may have superficial knowledge of the extent of the interaction between twins, we have no knowledge of the type and quality of the interaction. Moreover, we do not know whether any early environmental differences existed between MZ and DZ twins, or whether there were differences in parental behavior that could account for present differences. We hope to address these issues in future analyses.

REFERENCES

1. Hughes JR. Genetics of smoking: A brief review Behavior Therapy , 17 : 335-345 1986

2. Kaprio J Koskenvuo M Sarna S Cigarette smoking, use of alcohol, and leisure-time physical activity among same-sexed adult male twins In: Gedda L, et al.(Eds.) Twin Research 3: Epidemiological and Clinical Studies . New York : Alan R. Liss pp. 37-46 1981

3. Partanen J Brunn K Markkanen T Inheritance of Drinking Behavior: A Study of Intelligence, Personality and Use of alcohol of Adult Twins . Oslo : The Finnish Foundation for Alcohol Studies 1966

4. Pedersen N Twin similarity for usage of common drugs In: Gedda L Parisi P Nance WE (Eds.), Twin Research 3: Part C, Epidemiological and Clinical Studies . New York : Alan R. Liss pp. 53-59 1981



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