Adolescent Drinking Behavior and Genes : The Initiation of Alcohol Use

by National Institute of Health

(Page 3 of 5)

Twin studies consistently have demonstrated that initiation of alcohol use is largely influenced by environmental rather than genetic factors, a conclusion that is supported by the results from FinnTwin, which addressed this issue. Nearly two-thirds of the twins in this study reported never having used alcohol at age 14, whereas 36 percent reported that they had begun drinking alcohol at this age. Comparisons of the MZ and DZ twin pairs in the study determined that the environmental factors shared by co-twins played the largest role in influencing abstinence/drinking at age 14. These factors - which included the familial environment and the nonfamilial environments of peers, schools, and neighborhoods - accounted for 76 percent of the variation in drinking initiation in both boys and girls.

One relevant question in this context is whether the same factors influence drinking initiation in 14-year-old boys and girls. Although the magnitude of importance of environmental factors is the same in boys and girls, it is possible that different environmental factors may be acting in each gender. Twin studies allow researchers to estimate the extent to which the same or different environmental factors influence boys and girls. In the FinnTwin study, 77 percent of the environmental factors that contribute to the twin siblings' similarity in age of drinking initiation are the same for adolescent males and females (i.e., the correlation between environmental factors was estimated at 0.77). Thus, although many environmental influences impacting drinking initiation are the same for girls and boys, some unidentified environmental influences are specific to each gender.

Although the FinnTwin data provided little evidence of genetic effects on drinking initiation by age 14, some gender differences were apparent in that respect as well. Genetic effects were found to have a modest influence on girls (accounting for 18 percent of the variance in girls' drinking at age 14) but no influence on boys' drinking at the same age. Perhaps this difference reflects the fact that girls are more developmentally mature at age 14 than boys. By this age, most girls already have undergone significant pubertal development, whereas most boys are just beginning to experience pubertal changes. Girls' more advanced maturation may foster associations with older friends and peers that offer greater access to alcohol, thereby creating opportunities for girls to express drinking-related genetic predispositions at an earlier age.

Symptoms of Alcohol Dependence at Age 14

It is uncommon for 14-year-olds to exhibit symptoms of alcohol dependence, and at this age such symptoms are not attributable to genetic effects. To explore the issue of alcohol dependence at this early age in more detail, researchers administered a structured face-to-face interview to a sample of more than 1,800 twins from the FinnTwin study. Only 12.4 percent of the 14-year-old twins in this sample reported any symptoms of alcohol dependence, and only 1 percent met the criteria for a diagnosis of alcohol dependence. Moreover, more girls than boys reported alcohol dependence symptoms or met diagnostic criteria; this gender difference may be attributable to the fact that, at this age, girls are more likely than boys to be drinking.

For comparison, the investigators also determined the frequency of symptoms of conduct disorder in the same sample of twins. This analysis yielded several results that differed sharply from those obtained regarding alcohol dependence symptoms. Symptoms of CD were common among the adolescent twins: 44 percent of the sample reported one or more symptoms of CD, and 12 percent met diagnostic criteria for the condition. CD symptoms were more common among boys, who accounted for 65 percent of all twins diagnosed with CD. Genetic factors played a much different role in CD than in alcohol dependence. Whereas genetic factors had a negligible influence on alcohol dependence, they had a substantial influence on symptoms of CD.

Despite these differences, the two sets of symptoms are significantly correlated, meaning that the CD symptoms were more common among twins with a diagnosis of alcohol dependence and, similarly, the symptoms of alcohol dependence were more common among twins diagnosed with CD. However, because the twin data produced no evidence that genetic factors contribute to alcohol dependence symptoms at this age, the association of alcohol symptoms with CD symptoms must be attributed entirely to environmental factors that influence both conditions. One inference from these results is that CD is an early manifestation of a genetic predisposition that later contributes to the development of alcohol-related problems and alcohol dependence. Therefore, efforts to identify adolescents at risk for developing alcoholism and to create opportunities for targeted interventions should focus on identifying adolescents exhibiting symptoms of CD. Other research designs, such as studies of adopted children and their biological and adoptive families, also may yield important information on the genetic relationship between CD and alcohol dependence.

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