Alcohol and drug problems: a multivariate behavioural genetic analysis of co-morbidity.

Multivariate biometrical genetic analyses of self-report questionnaire items assessing problem alcohol and drug use were performed on data obtained from a sample of 438 volunteer twin pairs (236 monozygotic twin pairs, 247 dizygotic twin pairs). Additive genetic influences were moderate for all alcohol abuse items (21-46%), frequency of drug use (32%) and illicit drug use (32%). Prescribed drug use and debilitating drug use were largely environmentally determined (86% and 94%, respectively). The influence of environmental factors that influence all members of a family to the same degree (shared family environment) on each item was generally small (0-20%), whereas the influence of environmental factors unique to each family member (non-shared environment) comprised over half of the total variance on all items. Genetic factor analyses identified three uncorrelated common genetic factors. The first genetic factor appears to represent problems associated with alcohol and drug use, such as the inability to fulfil obligations at home, work or school. The second genetic factor is more specific to drug use and represents a general liability towards drug use, illicit or otherwise. The third genetic factor is specific to the alcohol use items only. The observed co-morbidity of alcohol and drug misuse can be attributed largely to a non-shared environmental factor common to both domains. Genetic co-morbidity appears to be limited to alcohol and substance misuse behaviours that interfere with normal daily functioning.