MedWire News: Results from a systematic review and meta-analysis of published studies confirm that a low IQ is associated with an increased risk for schizophrenia.
"Identification of premorbid IQ deficits in people who will later develop schizophrenia is a key piece of evidence underpinning a developmental aspect to the disorder," explain Golam Khandaker (University of Cambridge, UK) and team.
They add: "This has been summarized as a neurodevelopmental theory, for which there is empirical evidence.
"However, fundamental questions have yet to be settled. These include the absolute magnitude of premorbid IQ deficit, whether this is due to a decrement in the majority of future cases (a left-shift of this population), or whether it is due to a minority effect driven by a sub-group with conspicuously low IQ."
To investigate further, the team searched the literature for population-based or nested case-control studies that assessed IQ before the onset of schizophrenia.
In total, 12 studies, involving 4396 schizophrenia patients and 745,720 controls, met criteria for inclusion in the review.
Analysis of the pooled results confirmed that schizophrenia patients had a significantly lower premorbid IQ compared with mentally healthy controls, at an effect size of -0.43. This equates to a mean IQ score of 93.6 among individuals who later develop schizophrenia compared with a population mean of 100.0.
However, there was significant heterogeneity in effect size between studies, the researchers note.
Further analysis showed a "dose-response" effect, with each 1-point decrease in IQ associated with a significant 3.7% increase in the risk for schizophrenia.
The researchers also found that greater premorbid IQ deficits were associated with an earlier age at schizophrenia onset.
However, they found no evidence to suggest that IQ levels progressively decrease between the premorbid phase and illness onset.
Khandaker and team conclude: "Strong associations between premorbid IQ and risk for schizophrenia, and age of illness onset, argue for a widespread neurodevelopmental contribution to schizophrenia that operates across the entire range of intellectual ability.
"This also suggests higher IQ may be protective in schizophrenia, perhaps by increasing active cognitive reserve."
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