Many mental illnesses occur on a continuum with many more people showing at least some symptoms of them without being diagnosed with the full-blown condition. Several studies have indicated that psychosis could fall into this category with some people in 'non-clinical' populations displaying psychosis-like symptoms. A study of 6,455 12-year-old children in Avon looked into the prevalence of psychotic symptoms (auditory and visual hallucinations, and delusions) and how they were linked to IQ. There is a link between IQ and the risk of schizophrenia (the risk rises as IQ falls) but the relationship between IQ and psychosis-like symptoms is not clearly established. The researchers found that in the last 6 months 13.7% of the children had had at least one psychotic symptom. Those who had below-average IQ scores had an increased risk of such symptoms but for children of average, or above-average IQ there was no link between IQ and psychosis-like symptoms.
Horwood, Jeremy ... [et al] - IQ and non-clinical psychotic symptoms in 12-year-olds: results from the ALSPAC birth cohort British Journal of Psychiatry September 2008, 193(3), 185-191