title: Monozygotic twins discordant for major depression: a preliminary exploration of the role of environmental experiences in the aetiology and course of illness.
abstract: BACKGROUND
  Genetic effects upon behaviour are pervasive. To what extent are the many correlates of major depression (MD) due to individual-specific environmental experiences versus genetic factors correlated with risk for MD?
  
  
  METHODS
  From a population-based twin registry, we identified 72 female monozygotic pairs discordant for a lifetime history of MD and compared the affected and unaffected members on a wide range of putative correlates of MD.
  
  
  RESULTS
  The affected twin differed from her unaffected co-twin on many variables, eight of which were maximally discriminating: (i) maternal protectiveness; (ii) conflictual parent-child relationship; (iii) low optimism; (iv) current stressful life events; (v) financial difficulties and a history of (vi) phobia, (vii) nicotine dependence; and (viii) divorce. A cluster analysis suggested three 'environmental pathways' to MD characterized by: (i) childhood vulnerability and anxiety; (ii) acting-out and demoralization; and (iii) interpersonal difficulties.
  
  
  CONCLUSIONS
  Important precursors and sequelae of MD originate in environmental experiences unique to the individual and are not mediated through genetic factors or family-of-origin effects. Such environmental factors cause pervasive differences in monozygotic twins discordant for MD, especially in the areas of interpersonal difficulties, psychopathology, social problems and self-concept. These findings should be interpreted in the context of possible retrospective recall bias and the difficulty of distinguishing risk factors from sequelae in co-twin-control studies.
authors: Department of Psychiatry, Medical College of Virginia of Virginia Commonwealth University and Virginia Institute for Psychiatric and Behavioral Genetics, Richmond, USA.
authors: K S
authors: KS
authors: Kendler
authors: C O
authors: CO
authors: Gardner
mesh_terms: D000328:Adult; D000428:Alcohol Drinking / Q000453:epidemiology; D016000:Cluster Analysis; D003865:Major Depressive Disorder / Q000453:epidemiology / Q000209:etiology* / Q000235:genetics*; D004777:Environment; D005260:Female; D006304:Health Status; D006801:Humans; D017533:Marital Status; D010287:Parent-Child Relations; D010551:Personality; D011569:Psychiatric Status Rating Scales; D012307:Risk Factors; D012649:Self Concept; D012720:Severity of Illness Index; D019966:Substance-Related Disorders / Q000453:epidemiology; D014430:Twins, Monozygotic / Q000523:psychology*