Adolescent Drinking Behavior and Genes

Influences on Drinking and Patterns
by National Institute of Health

(Page 4 of 5)

Identifying Specific Environmental Influences on Drinking

The environmental factors shared by siblings, and assessed in twin studies such as FinnTwin, include not only household environmental characteristics (such as family structure and status, parenting practices, and home atmosphere) but also environmental influences exerted through schools, neighborhoods, and communities. For some behaviors and at some ages, the nonfamilial experiences that children share with their peers at school and in their neighborhoods may be more salient than the household influences they share with their families in their homes. Drinking in early adolescence may be one such behavior. To evaluate this hypothesis, researchers can assess the contribution of nonfamilial environments to children's behavior by comparing the resemblance of unrelated classmates of twin siblings to one another and to their classmate twins.

The FinnTwin study undertook such an approach. For this analysis, researchers linked each twin with a classmate control subject of the same gender who had the same year of birth, lived in the same neighborhood, and attended the same school and classroom as the twin.

As discussed earlier, behavioral resemblance of co-twins results from their shared genetic makeup and from the household experiences they share with each other and with their parents, as well as experiences they share with peers outside the home. In contrast, any behavioral resemblance between the twins and their classmates (who are genetic strangers and share no household experiences with the twins being studied) must result entirely from experiences shared in school and neighborhood. Accordingly, this research approach provides a direct estimate of the role of environmental influences associated with schools and neighborhoods.

In the FinnTwin study, behavior of the twins and the classmate control subjects was assessed when the children were 11 or 12 years old by means of a questionnaire administered during school hours under research supervision. The questionnaire included three drinking-related items illustrating nonfamilial environmental effects:

• "Have you ever drunk alcohol?"
• "Have you ever seen any of your friends drunk?"
• "Have you ever drunk alcohol with your friends without adults around?"

The prevalence of affirmative answers to these questions differed widely, as would be expected among children at this age. For example, more than two-thirds of the boys and more than half of the girls admitted to having had a drink of alcohol - which is not uncommon in Finnish family culture, where children may have a drink at holiday time and with parental knowledge. In contrast, only 18 percent of boys and 11 percent of girls reported having seen friends drunk, and only 7 percent of the boys and girls reported ever having drunk alcohol with their friends without adults around.

The investigators determined the relative influences of genetic and specific environmental factors on these three variables based on the relative similarities of MZ and DZ twins and their classmate control subjects. They found that genetic effects accounted for only 8 percent of the individual differences in whether the children had begun to use alcohol, but they accounted for 30 percent of the variance in alcohol use without adult supervision. Conversely, familial effects were very influential in initiation of use, accounting for nearly 40 percent of the variance. They had little influence, however, on whether these children had ever seen any of their friends drunk, which was more closely linked to genetic influences and the effects of the child's personal experiences (i.e., experiences not shared with the twin sibling). Finally, experiences shared with school classmates contributed significantly to each of these alcohol-related experiences.

Thus, this expanded twin - classmate control design has identified important influences on drinking behavior that originate both within and outside the family environment. These findings document the varying influences of a person's genetic dispositions on different aspects of drinking behavior in early adolescence.

Drinking Patterns across Adolescence

The relative contributions of genetic and environmental factors to the drinking behavior of young people change during adolescence. As previously described, familial and environmental influences predominate in their impact on initiation of drinking behavior. When people move from early experimental alcohol use to more established patterns of use, however, the importance of individual genetic predispositions increases greatly, and the importance of common environmental factors decreases accordingly. Findings from the FinnTwin study illustrate this process.

One component of FinnTwin was an assessment of the twins at ages 16, 17, and 18.5. Over this 30-month period of followup, the number of abstinent participants decreased dramatically, from about 25 percent of twins at age 16 to only about 10 percent of twins at age 18.5. Conversely, the proportion of twins who reported drinking at least twice monthly more than doubled over the 30-month period, and at age 18.5 more than 60 percent of twin boys and girls drank that often.

Interestingly, the influence of genetic factors also increased across this age range. Genetic factors accounted for only 33 percent of the variation in drinking frequency at age 16, but 50 percent of the variation by age 18. At the same time, common environmental factors became less important, accounting for 37 percent of the variance at age 16, but only 14 percent by age 18. This shift in the relative influence of environmental factors is even more dramatic when one includes the data on drinking initiation from 14-year-old twins in the FinnTwin study. Taken together, these findings indicate that familial and community factors appear to have their greatest impact on adolescents' decisions to initiate alcohol use and on their early experimental drinking; as drinking patterns develop, differentiate, and stabilize across adolescence, however, genetic influences on drinking patterns become increasingly important.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.