Specificity of familial risk for alcoholism versus major depression

A number of hypotheses have been advanced to explain comorbidity observed between alcoholism and depression. We administered diagnostic interviews to 3372 pairs of male twins from the Vietnam Era Twin Registry by tele-phone. Twins were assessed for DSM-III-R major depression (MD) and alcohol dependence and provided information about parental alcoholism and depression. There was a substantial phenotypic association between alcoholism and MD. MD in one twin was associated with risk of MD alone and MD plus alcoholism, but not alcoholism alone in the co-twin. Alcoholism in one twin was associated with risk of alcoholism alone and alcoholism plus MD, but not MD alone in the co-twin. The same pattern was observed for parent-offspring associations. Our data suggest that alcoholism is a risk factor for MD and MD is a risk factor for alcoholism, but in men, familial (i.e., genetic and family environmental) vulnerability for each disorder is distinct. The co-occurrence of alcoholism and MD is associated with greater clinical severity of each. Symptoms of alcoholism usually preceded onset of MD, but some individuals had onset of MD before developing any symptoms of alcoholism. Individuals with a lifetime history of both alcoholism and MD had more severe alcoholism and depression and earlier.