Background: Alcohol misuse and conduct disorder (CD) are strongly associated in adolescents. Is their association due to shared genetic and environmental liabilities? We addressed that question with data obtained from structured interviews of 14-year-old Finnish twins.
Methods: A total of 1854 twins completed face-to-face interviews. Univariate models, allowing for sex limitation, were fit to symptom counts for Diagnostic and Statistical Manual of Mental Disorders, Third Edition, Revised diagnoses of CD and alcoholism to examine their genetic and environmental influences. Then, bivariate models evaluated the extent to which genetic and environmental influences contributed to the covariation between symptom counts for the two disorders.
Results: A total of 822 twins (44% of the sample) reported one or more symptoms of CD; alcohol dependence symptoms were much less common, reported by only 12%. The correlation between the two symptom counts was 0.50. Models fit to the twin data demonstrated that CD symptoms were under significant genetic influence in both boys and girls, although those influences were predominantly sex specific. In contrast, alcohol dependence symptoms were, at this age, under sex-specific effects of common environments, with no evidence of genetic influences. Accordingly, the substantial covariation between alcohol dependence symptoms and those of CD was attributed entirely to shared environmental effects.
Conclusions: At age 14, genetic influences on alcohol dependence symptoms are negligible, and the correlated liabilities between these symptoms and those of CD are to be found in environmental factors that are common to both.