THE INFLUENCE OF FAMILIAL AND NONFAMILIAL FACTORS ON THE ASSOCIATION BETWEEN MAJOR DEPRESSION AND SUBSTANCE-ABUSE DEPENDENCE IN 1874 MONOZYGOTIC MALE TWIN PAIRS

The co-occurrence of major depression (MD) with alcohol and illicit su bstance abuse/dependence (A/D) has been repeatedly observed. However, prior research has been unable to determine whether or not the co-occu rrence is a result of familial vulnerability or non-familial influence s. The present study examines the association of the lifetime diagnose s of MD with alcohol, cannabis, amphetamine, cocaine, and sedative A/D (DSM-III-R criteria) before and after controlling for familial factor s in a non-clinical sample of 1874 middle aged, monozygotic male twin pairs. A lifetime diagnosis of MD was significantly associated with li fetime diagnoses of alcohol and illicit substance A/D prior to account ing for familial factors (odds ratios: 1.8-4.5). After employing a co- twin analytical technique to control for familial factors, a lifetime diagnosis of MD remained significantly associated only with lifetime d iagnoses of cannabis, amphetamine and sedative A/D (odds ratios: 2.3-1 0.9). These results suggest that the association between MD and alcoho l A/D is influenced by familial factors. In contrast, the association between MD and illicit substances of A/D is largely explained by non-f amilial factors.