title: A twin study of genetic relationships between psychotic symptoms.
abstract: OBJECTIVE
  Biometrical model fitting was applied to clinical data from twins to investigate whether operationally defined schizophrenic, schizoaffective, and manic syndromes share genetic risk factors.
  
  
  METHODS
  Seventy-seven monozygotic and 89 same-sex dizygotic twin pairs in which the proband met the Research Diagnostic Criteria (RDC) for lifetime-ever schizophrenic, schizoaffective, or manic syndrome were ascertained from the Maudsley Twin Register in London. The syndromes were defined non-hierarchically. Correlations in liability were calculated for each syndrome in the monozygotic and dizygotic pairs and across the three pairings of schizophrenic-manic, schizophrenic-schizoaffective, and schizoaffective-manic syndromes both within probands and within pairs. For the three syndromes considered together, an independent pathway model was fitted.
  
  
  RESULTS
  The model fitting showed significant genetic correlations between all three syndromes. There was evidence of both common and syndrome-specific genetic contributions to the variance in liability to the schizophrenic and manic syndromes, but the genetic liability to the schizoaffective syndrome was entirely shared in common with the other two syndromes. In contrast, environmental liability to the schizoaffective syndrome was not shared with the other syndromes.
  
  
  CONCLUSIONS
  If diagnostic hierarchies are relaxed, there is a degree of overlap in the genes contributing to RDC schizophrenic, schizoaffective, and manic syndromes. Supplementing the traditional approach of assigning a single main lifetime diagnosis with information on within-person comorbidity of psychotic syndromes may provide valuable information about the familial aggregation of psychotic symptoms.
authors.affiliation: Department of Psychological Medicine, University of Wales College of Medicine, Cardiff, UK.
authors.forename: Alastair G
authors.initials: AG
authors.lastname: Cardno
authors.forename: Frühling V
authors.initials: FV
authors.lastname: Rijsdijk
authors.forename: Pak C
authors.initials: PC
authors.lastname: Sham
authors.forename: Robin M
authors.initials: RM
authors.lastname: Murray
authors.forename: Peter
authors.initials: P
authors.lastname: McGuffin
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001714:Bipolar Disorder / Q000175:diagnosis / Q000235:genetics* / Q000523:psychology; D004200:Diseases in Twins; D005260:Female; D020022:Genetic Predisposition to Disease / Q000235:genetics; D006801:Humans; D008131:London; D008297:Male; D008875:Middle Aged; D008957:Models, Genetic; D011618:Psychotic Disorders / Q000175:diagnosis / Q000235:genetics* / Q000523:psychology; D012042:Registries; D018570:Risk Assessment; D012559:Schizophrenia / Q000175:diagnosis / Q000235:genetics*; D012565:Schizophrenic Psychology; D012931:Social Environment; D014429:Twins, Dizygotic / Q000235:genetics; D014430:Twins, Monozygotic / Q000235:genetics