title: Adolescent twins discordant for major depressive disorder: shared familial liability to externalizing and other internalizing disorders.
abstract: BACKGROUND
  In adolescents, as in adults, there is often comorbidity between major depressive disorder (MDD) and many other disorders. In this study, the discordant twin method was used to investigate whether this comorbidity in adolescents may be due to a shared familial liability between MDD and other internalizing disorders, and between MDD and externalizing disorders.
  
  
  METHODS
  We examined prevalence rates of anxiety disorders, substance use disorders, and childhood externalizing disorders in 624 seventeen-year-old same-sex monozygotic and dizygotic twin pairs. Overall prevalence of MDD in this sample was 12.9%. Twenty-seven twin pairs were concordant for MDD, 107 were discordant for MDD, and 490 were concordant for no MDD.
  
  
  RESULTS
  Prevalence rates for disorders other than MDD were elevated in the depressed twins relative to the control twins from pairs concordant for no MDD. Prevalence rates were also elevated in the nondepressed co-twins of depressed twins. Twin concordance for disorders other than MDD was higher in twin pairs where at least one twin was depressed than in pairs where neither twin was depressed.
  
  
  CONCLUSIONS
  These results support a shared familial liability between MDD and other internalizing disorders and between MDD and externalizing disorders in adolescents.
authors.affiliation: University of Minnesota-Twin Cities, USA.
authors.forename: Stephanie L
authors.initials: SL
authors.lastname: Burcusa
authors.forename: William G
authors.initials: WG
authors.lastname: Iacono
authors.forename: Matt
authors.initials: M
authors.lastname: McGue
mesh_terms: D000293:Adolescent; D000294:Adolescent Behavior; D001008:Anxiety Disorders / Q000235:genetics* / Q000523:psychology; D015897:Comorbidity; D003866:Depressive Disorder / Q000235:genetics* / Q000523:psychology; D005192:Family Health; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D008297:Male; D019966:Substance-Related Disorders / Q000235:genetics* / Q000523:psychology; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic