IQ change over time in schizophrenia and healthy individuals: A meta-analysis

Anna M. Hedman*, Neeltje E.M. van Haren, Caroline G.M. van Baal, René S. Kahn, Hilleke E. Hulshoff Pol

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

56 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)201-208
Number of pages8
JournalSchizophrenia Research
Issue number1-3
Publication statusPublished - May 2013
Externally publishedYes

Bibliographical note

Funding Information:
This work was supported by NWO VIDI grant (grant no.: 917-46-370) (to H.H.) and European Twin Study Network on Schizophrenia grant (EUTwinsS); (grant no.: MRTN-CT-2006-035987).


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