title: IQ change over time in schizophrenia and healthy individuals: a meta-analysis.
abstract: BACKGROUND
  Cognitive deficits have been recognized as a key feature of schizophrenia, since the first description by Kraepelin. Specifically, lower intelligence is considered a core feature of the disorder and may represent a risk factor for its development. However, whether global intelligence decreases over time in schizophrenia is not known. The aims of this quantitative meta-analysis are to gather, integrate and estimate the overall mean effect size of IQ change over time in schizophrenia as compared to healthy individuals.
  
  
  METHODS
  A systematic search was conducted to identify relevant studies. Longitudinal studies with at least two intelligence assessments in schizophrenia cohorts were retrieved. Studies had to report sufficient data on IQ-change and include data from healthy comparisons for computation of effect sizes. For each study, the Cohen d was calculated as well as a combined mean effect size.
  
  
  RESULTS
  Fourteen studies were identified. Eight studies with a total of 280 patients and 306 healthy controls were suitable to be included. The mean weighted baseline IQ was 97.20 for patients and 109.26 for controls. The mean weighted IQ-change per year was +0.33 for patients and +2.08 for controls. The combined effect size was Cohen's d = -0.48, p = 0.01.
  
  
  CONCLUSIONS
  A global cognitive deficit is present in patients with schizophrenia expressed as a lower test score increase over repeated testing as compared to healthy subjects possibly due to the lack of practice effects in patients. Thus, schizophrenia is characterized by a relative lack of gain in global cognitive abilities over time.
authors.affiliation: Rudolf Magnus Institute of Neuroscience, Department of Psychiatry, University Medical Centre Utrecht, PO Box 85500, 3508 GA Utrecht, The Netherlands. A.Hedman@umcutrecht.nl
authors.forename: Anna M
authors.initials: AM
authors.lastname: Hedman
authors.forename: Neeltje E M
authors.initials: NE
authors.lastname: van Haren
authors.forename: Caroline G M
authors.initials: CG
authors.lastname: van Baal
authors.forename: René S
authors.initials: RS
authors.lastname: Kahn
authors.forename: Hilleke E
authors.initials: HE
authors.lastname: Hulshoff Pol
mesh_terms: D003072:Cognition Disorders / Q000209:etiology*; D015331:Cohort Studies; D016208:Databases, Factual / Q000706:statistics & numerical data; D005260:Female; D006801:Humans; D007360:Intelligence / Q000502:physiology*; D007361:Intelligence Tests; D008297:Male; D012559:Schizophrenia / Q000150:complications*; D012565:Schizophrenic Psychology