Background: Although prior family and twin studies have examined the relationship between the genetic and environmental risk factors for pairs of psychiatric disorders, the interrelationship between these classes of risk factors for a broad range of psychiatric disorders remains largely unknown.
Methods: An epidemiologic sample of 1030 female-female twin pairs with known zygosity, ascertained from the Virginia Twin Registry, were evaluated by a personal interview conducted by mental health professionals, assessing lifetime history of phobia, generalized anxiety disorder, panic disorder, bulimia nervosa, major depression, and alcoholism.
Results: A multivariate twin analysis suggested the following. First, genetic, familial-environmental, and individual-specific environmental risk factors each cause a unique pattern of comorbidity among the six disorders. Second, genetic influences on these disorders are best explained by two factors, the first of which loads heavily on phobia, panic disorder, and bulimia nervosa and the second, on major depression and generalized anxiety disorder. Third, unlike other disorders, genetic influences on alcoholism are largely disorder specific. Fourth, familial-environmental influences on these disorders are best explained by a single factor that substantially influenced liability to bulimia nervosa only. Fifth, individual-specific environmental influences on the risk for these psychiatric disorders are best explained by a single factor, with highest loadings on generalized anxiety disorder and major depression and with large-disorder-specific loadings, especially on phobias, panic disorder, and alcoholism.
Conclusions: These results support the following hypotheses: First, each major risk factor domain (genes, family environment, and individual-specific environment) influences comorbidity between these disorders in a distinct manner. Second, genetic influences on these six disorders are neither highly specific nor highly nonspecific. Neither a model that contains a discrete set of genetic factors for each disorder nor a model in which all six disorders results from a single set of genes is well supported. Third, the anxiety disorders are not, from a genetic perspective, etiologically homogeneous. Fourth, most of the genetic factors that influence vulnerability to alcoholism in women do not alter the risk for development of other common psychiatric disorders. These results should be interpreted in the context of both the strengths and limitations of multivariate twin analysis.