Despite the findings suggestive of a significant role for genetic factors, behavioral scientists seem generally reluctant to acknowledge the contribution of these factors to appetitive behaviors that fall within the “normal” range of consumption. For example, neither the 1988 nor the 1989 Surgeon General's Report 5 , 6 mentions any of the studies cited above that show small but significant heritable components to smoking behavior. The reluctance to acknowledge the results from these studies may be related to the belief that genetic determinants imply a lack of modifiability. However, if genetic effects do play a role in smoking behavior, further studies that sort out these effects could lead to an increased understanding of why there continues to be a relapse problem among smokers who have recently quit, despite the most ardent and forceful efforts of public health specialists to change these behaviors.
In the present paper, we hope to demonstrate the potential value of studying twins in general and, more specifically, of the NAS-NRC World War II Twin Registry, for examining issues related to the genetics of substance use. It is our belief that twin studies actually provide valuable opportunities for investigating the contribution of environmental influences on substance use behavior. For example, by comparing the life-long smoking behavior of genetically identical MZ twin pairs, it is possible to eliminate genetic variability, thereby creating a pure culture of nongenetic determinants--determinants that can be identified as being “environmental exposures” and “personal behaviors.” Once we have established that such environmental exposures during adult life are responsible for a behavior in question, appropriate changes in the environment could be recommended as an effective means of intervention.
Apart from the power of the twin method to assess the roles of genetic and environmental factors of health behaviors during late adulthood, the NAS-NRC Twin Registry is noteworthy for several specific features. First, the cohort is sufficiently large to support powerful analyses; second, excellent longitudinal data are available on these subjects from military induction (ages 17 to 28 years) and during middle age; and, third, repeated information on an array of cardiovascular disease risk factors and health behaviors was collected on these subjects when they were 42 to 55 years old and repeated 10 years later by the administration of standardized epidemiological questionnaires. It is our intent in this report to present some preliminary results from a genetic analysis of smoking, alcohol use, and coffee consumption in the NAS-NRC Twin Registry. Our results support the conclusion that genetic factors contribute to the variation in substance use behaviors in late adulthood and that these behaviors are minimally influenced by shared environmental factors and the simultaneous occurrences of combinations of these behaviors.