Indicators of genetic and environmental influences in drug abusing individuals
To assess relative contribution of genetic and environmental influences in individual cases of drug abuse/dependence, separate scales were constructed from DIS (Version III) drug symptom items. Using data from 38 MZ and 35 DZ male twin pairs, items with significant MZ/DZ differences in probandwise concordance were assigned to a genetic scale whereas items without significant MZ/DZ differences were assigned to an environmental scale. As expected, significant differences were found between MZ and DZ twins in intraclass correlations for the genetic but not environmental scale. Genetic scores on drug and alcohol scales were correlated (r = 0.40), whereas environmental scores were not. When scores on the genetic scales were compared, the correlation between drug and alcohol scores within individuals was similar to that across MZ twins, both of which were higher than the correlation across DZ twins. These results suggest (1) relative contribution of genetic and environmental influences in individual cases of drug dependence can be assessed, and (2) common genetic mechanisms may be involved in alcohol and drug dependence.