title: A pilot Swedish twin study of affective illness, including hospital- and population-ascertained subsamples.
abstract: OBJECTIVE
  We sought to compare the probandwise concordance rate (PRC) for affective illness (AI) in monozygotic (MZ) and dizygotic (DZ) twins in samples ascertained through psychiatric hospitalization vs samples from the general population.
  
  
  METHODS
  Twins were ascertained through psychiatric hospitalization for AI from the Swedish Psychiatric Twin Registry or as a matched sample from the population-based Swedish Twin Registry. Lifetime diagnoses were based on a mailed questionnaire containing, in self-report format, DSM-III-R criteria for mania and major depression. Returned questionnaires were obtained from 1484 individuals and both members of 486 pairs, of whom 154 were classified as MZ, 326 as DZ, and six of unknown zygosity.
  
  
  RESULTS
  No evidence was found for violations of the equal environment assumption. Using either a narrow or broad diagnostic approach, the risk for AI in cotwins of proband twins was independent of the gender, polarity (ie, unipolar vs bipolar) and mode of ascertainment of the affected proband (ie, via hospitalization vs from the general population). Combining both subsamples, PRC for total AI using narrow diagnostic criteria was 48.2% in MZ and 23.4% in DZ twins. Using broad diagnostic criteria, the parallel figures were 69.7% and 34.9%. The risk for bipolar illness was substantially increased in the cotwins of probands with bipolar AI.
  
  
  CONCLUSIONS
  Genetic factors play a major role in the etiology of AI in Sweden, as assessed by self-report questionnaire. Heritable factors appear to be equally important in AI as ascertained in clinical and epidemiological samples.
authors.affiliation: Department of Psychiatry, Medical College of Virginia/Virginia Commonwealth University, Richmond.
authors.forename: K S
authors.initials: KS
authors.lastname: Kendler
authors.forename: N
authors.initials: N
authors.lastname: Pedersen
authors.forename: L
authors.initials: L
authors.lastname: Johnson
authors.forename: M C
authors.initials: MC
authors.lastname: Neale
authors.forename: A A
authors.initials: AA
authors.lastname: Mathé
mesh_terms: D001714:Bipolar Disorder / Q000175:diagnosis / Q000453:epidemiology / Q000235:genetics; D003866:Depressive Disorder / Q000175:diagnosis / Q000453:epidemiology* / Q000235:genetics; D004200:Diseases in Twins / Q000175:diagnosis / Q000453:epidemiology* / Q000235:genetics; D005260:Female; D006753:Hospital Records; D006778:Hospitals, Psychiatric; D006801:Humans; D008297:Male; D008875:Middle Aged; D010865:Pilot Projects; D012307:Risk Factors; D011795:Surveys and Questionnaires; D013548:Sweden / Q000453:epidemiology; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic