title: A population-based study of shared genetic variation between premorbid IQ and psychosis among male twin pairs and sibling pairs from Sweden.
abstract: BACKGROUND
  The strong association between lower IQ and risk for psychosis has led to the suggestion that the search for genes influencing cognition may provide a useful strategy for examining the genetic origins of psychosis. However, research in this area has generally used designs in which twin pairs are selected by case status and with assessment of IQ after the onset of psychosis rather than longitudinal population-based samples.
  
  
  OBJECTIVE
  To examine the relationship and shared genetic origin between premorbid IQ and psychotic disorders in a longitudinal population-based cohort.
  
  
  METHODS
  Genetically informative longitudinal study.
  
  
  METHODS
  Population-based cohort in Sweden.
  
  
  METHODS
  Individuals were identified from the population-based Swedish Multi-Generation Register and consisted of male sibling (n = 369 960), monozygotic twin (n = 1986), and dizygotic twin (n = 2253) pairs born between January 1951 and December 1976. Their IQs were measured during compulsory military conscription.
  
  
  METHODS
  Individuals having a subsequent diagnosis of psychosis were identified via the Swedish National Hospital Discharge Register.
  
  
  RESULTS
  Heritability estimates for IQ and psychosis were similar to previous estimates, approximately 69% and 56%, respectively. However, the phenotypic correlation between IQ and psychosis was only -0.11, of which 91% was due to shared genetic influences. The proportion of genetic variance for psychosis shared with that for IQ was approximately 7%.
  
  
  CONCLUSIONS
  Using IQ as a phenotype to identify genes that have an important role in the genetic origin of schizophrenia is unlikely to be a successful strategy. The low correlation seen in this study between premorbid IQ and psychosis vs the higher correlations reported in the literature with postmorbid IQ suggests the correlation between these phenotypes has more to do with the influence that the onset of psychosis has on cognitive functioning than with shared genetic origin.
authors.affiliation: Unit of Urologic and Genetic Epidemiology, University of Birmingham, and Heart of Birmingham Teaching Primary Care Trust, Birmingham, England.
authors.forename: Tom
authors.initials: T
authors.lastname: Fowler
authors.forename: Stanley
authors.initials: S
authors.lastname: Zammit
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Owen
authors.forename: Finn
authors.initials: F
authors.lastname: Rasmussen
mesh_terms: D000328:Adult; D004200:Diseases in Twins / Q000235:genetics* / Q000523:psychology; D014644:Genetic Variation; D006801:Humans; D007360:Intelligence / Q000235:genetics* / Q000502:physiology; D007361:Intelligence Tests; D008137:Longitudinal Studies; D008297:Male; D011618:Psychotic Disorders / Q000235:genetics* / Q000503:physiopathology; D012042:Registries; D012307:Risk Factors; D012559:Schizophrenia / Q000235:genetics / Q000503:physiopathology; D035781:Siblings / Q000523:psychology; D013548:Sweden; D014429:Twins, Dizygotic / Q000523:psychology; D014430:Twins, Monozygotic / Q000523:psychology