Background: Although major depression (MD) and alcoholism co-occur in clinical and epidemiologic samples of women more often than expected by chance, the magnitude and causes of this comorbidity are uncertain.
Methods: Personal interviews were conducted with 2163 female twins from a population-based twin registry. Bivariate twin analysis was performed using two definitions of MD and three definitions of alcoholism of varying diagnostic breadth.
Results: Odds ratios ranged from 2.7 to 6.0 and were consistently higher using narrower diagnostic criteria for either disorder. Twin analyses found (1) no evidence for familial environmental factors for either MD or alcoholism; (2) significant genetic correlations, ranging from +.4 to +.6, between MD and alcoholism, which were higher using narrower criteria for alcoholism; (3) significant individual-specific environmental correlations, ranging from +.2 to +.4, for all but one of the diagnostic combinations, which were higher using narrower criteria for MD.
Conclusions: Comorbidity between MD and alcoholism in women is substantial and appears to result largely from genetic factors that influence the risk to both disorders, but common environmental risk factors also contribute. However, genetic factors exist that influence the liability to MD without influencing the risk for alcoholism and vice versa. Narrowing the diagnostic criteria for MD or alcoholism increases comorbidity, but for different reasons narrow diagnostic criteria for MD increase the environmental sources of comorbidity while narrow diagnostic criteria for alcoholism increase the genetic sources of comorbidity.