title: The sources of co-morbidity between major depression and generalized anxiety disorder in a Swedish national twin sample.
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.
  
  
  METHODS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Psychiatry Medical College of Virginia/Virginia Commonwealth University, Richmond, VA 23298-0126, USA. kendler@vcu.edu
authors.forename: Kenneth S
authors.initials: KS
authors.lastname: Kendler
authors.forename: Charles O
authors.initials: CO
authors.lastname: Gardner
authors.forename: Margaret
authors.initials: M
authors.lastname: Gatz
authors.forename: Nancy L
authors.initials: NL
authors.lastname: Pedersen
mesh_terms: D000328:Adult; D000368:Aged; D001008:Anxiety Disorders / Q000175:diagnosis / Q000453:epidemiology / Q000235:genetics* / Q000523:psychology; D002605:Character; D015897:Comorbidity; D003430:Cross-Sectional Studies; D003865:Major Depressive Disorder / Q000175:diagnosis / Q000453:epidemiology / Q000235:genetics* / Q000523:psychology; D004200:Diseases in Twins / Q000175:diagnosis / Q000453:epidemiology / Q000235:genetics* / Q000523:psychology; D005260:Female; D006801:Humans; D008297:Male; D008403:Mass Screening; D008875:Middle Aged; D015233:Models, Statistical; D009497:Neurotic Disorders / Q000175:diagnosis / Q000453:epidemiology / Q000235:genetics / Q000523:psychology; D012042:Registries; D012737:Sex Factors; D012931:Social Environment; D013223:Statistics as Topic; D011795:Surveys and Questionnaires