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Is there an association between cognitive function and risk for schizophrenia?
Diagnosis of schizophreniform, schizoaffective, schizotypal or psychotic disorders or schizophrenia, using ICD or DSM criteria.
Systematic review and meta-analysis.
Medline-Pubmed, PsycINFO and Embase were searched from 1984 up to February 2011. Hand searches of selected publications' reference lists and related conference proceedings were also conducted. Prominent researchers in the field were contacted for unpublished work.
Study selection and analysis
For inclusion studies had to be population-based cohort or nested case control studies that measured intelligence using standard psychometric tests in childhood or before the onset or diagnosis of schizophrenia, and identified cases using population-based registers or cohorts. Studies that used an estimation or indirect assessment of IQ were excluded. For populations reported in multiple publications, the publication with the longer follow-up or larger sample was selected. Measures of premorbid full scale, verbal and performance IQ were extracted, and differences in premorbid IQ between cases and controls were analysed as standardised mean difference (SMD). Dose-response relationship between premorbid IQ and risk of schizophrenia was assessed by defining six IQ groups (IQ <−2SD, −2 to −1SD, −1 to 0 SD, 0 to +1SD, +1 to …
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