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

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.