Origins and Consequences of Age at First Drink. II. Familial Risk and Heritability

BACKGROUND: The association of age at first drink (AFD) and alcoholism may reflect a common inherited vulnerability to disinhibitory behavior and psychopathology rather than a direct influence of the former on the latter. We tested the common-inherited-vulnerability hypothesis by determining whether AFD is familial and heritable. METHODS: A sample of 1,232 14-year-old twins was classified according to their biological parents' AFD. RESULTS: Lifetime symptoms of externalizing disorders (i.e., conduct disorder and oppositional defiant disorder) were significantly higher in the sons but not the daughters of parents whose AFD came before age 15 years. Offspring symptoms of internalizing disorders (i.e., major depressive disorder, separation anxiety disorder, and overanxious disorder) were not significantly associated with parental early use of alcohol in either sons or daughters. Early use of alcohol in mothers but not fathers was associated with a significant increase in both sons' and daughters' risk of alcohol use by age 14. The contribution of genetic and environmental factors to risk of early alcohol use was investigated in a sample of 416 monozygotic (MZ) and 225 like-sex dizygotic (DZ) 14-year-old twin pairs. Twin similarity for early alcohol use was substantially greater in MZ than DZ twins in boys but about equally similar in MZ and DZ twins in girls. Estimated heritability of early alcohol use was significantly greater in boys (55%) than girls (11%), in part because the genetic factors underlying symptoms of disinhibitory psychopathology contributed more to risk of early alcohol use in boys than girls. CONCLUSIONS: Early use of alcohol is familial and, at least in males, heritable. The familial transmission of early alcohol use is caused in part by genetic risk for disinhibitory psychopathology in males and to shared environmental factors in girls. These results provide some support for a common-inherited-vulnerability hypothesis, and suggest that the processes underlying early alcohol use may differ in boys and girls.