Psychological Medicine

Psychological Medicine (2007), 37:3:453-462 Cambridge University Press
Copyright © 2006 Cambridge University Press
doi:10.1017/S0033291706009135

Original Article

The sources of co-morbidity between major depression and generalized anxiety disorder in a Swedish national twin sample


KENNETH S. KENDLER a1a2c1, CHARLES O. GARDNER a1, MARGARET GATZ a3a4 and NANCY L. PEDERSEN a3a4
a1 Department of Psychiatry Medical College of Virginia/Virginia Commonwealth University, Richmond, VA, USA
a2 Department of Human Genetics, Medical College of Virginia/Virginia Commonwealth University, Richmond, VA, USA
a3 Department of Psychology, University of Southern California, USA
a4 Department of Medical Epidemiology and Biostatistics, Karolinska Institute, Stockholm, Sweden

Article author query
kendler ks   [PubMed][Google Scholar] 
gardner co   [PubMed][Google Scholar] 
gatz m   [PubMed][Google Scholar] 
pedersen nl   [PubMed][Google Scholar] 

Abstract

Background. Prior studies report high levels of co-morbidity between major depression (MD) and generalized anxiety disorder (GAD) and suggest that these disorders are closely related genetically. The personality trait of neuroticism (N) is substantially correlated with risk for MD and GAD.

Method. Bivariate twin models were applied to lifetime diagnoses of modified DSM-IV diagnosis of MD and GAD obtained at personal interview in 1998–2003 with 37296 twins from the population-based Swedish Twin Registry. A trivariate Cholesky model with N, MD and GAD was applied to a subset (23280 members of same-sex twin pairs) who completed a self-report questionnaire assessing N in 1972–1973.

Results. In the best-fit bivariate model, the genetic correlation between MD and GAD was estimated at +1·00 in females and +0·74 in males. Individual-specific environmental factors were also shared between the two disorders with an estimated correlation of +0·59 in males and +0·36 in females. In the best-fit trivariate Cholesky model, genetic factors indexed by N impacted equally on risk for MD and GAD in males and females. However, in both sexes, genetic risk factors indexed by N contributed only around 25% to the genetic correlation between MD and GAD.

Conclusion. Genetic risk factors for lifetime MD and GAD are strongly correlated, with higher correlations in women than in men. Although genetic risk factors indexed by the personality trait of N contribute substantially to risk for both MD and GAD, the majority of genetic covariance between the two disorders results from factors not shared with N.

(Published Online November 23 2006)


Correspondence:
c1 Virginia Institute for Psychiatric and Behavioral Genetics of Virginia Commonwealth University, Box 980126, Richmond, VA 23298-0126, USA. (Email: kendler@vcu.edu)


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