Indicators of genetic and environmental influence in alcohol-dependent individuals.

Although much is known about genetic and environmental influences in alcohol dependence at the population level, little is known about the relative contribution of such influences on individuals. As an initial step toward individual assessment, concordance for the Diagnostic Interview Schedule, version III alcohol symptoms was determined in a sample (n = 113) of male monozygotic (MZ) and dizygotic (DZ) twins. Items were assigned to a genetic or environmental scale on the basis of significant MZ/DZ differences in proband-wise concordance rates. Weights were assigned to items based on factor analyses. For the genetic scale, significant differences were found between MZ and DZ intraclass correlations. No significant differences were found between MZ and DZ correlations on the environmental scale. When scores on the environmental scale were controlled, genetic scale scores were correlated with earlier age of onset of alcohol problems and a shorter interval between first intoxication and onset of alcohol problems. When scores on the genetic scale were controlled, environmental scale scores were correlated with later age of onset of alcohol problems and a longer interval between first intoxication and onset of alcohol problems. These results suggest it is possible to assess relative influence of genetic and environmental factors in individual cases of alcohol dependence.