Associations between premorbid intellectual performance, early-life exposures and early-onset schizophrenia. Cohort study.

BACKGROUND Impaired intellectual performance is associated with an increased risk of schizophrenia. AIMS To investigate whether this association is due to the influence of prenatal and early childhood exposures on both intellectual development and the risk of schizophrenia. METHOD Cohort of 197 613 Swedish male conscripts with linked birth, census and hospital admission data together with five measures of verbal and non-verbal intellectual performance recorded at conscription. RESULTS 109 643 subjects had complete data; over a mean 5-year follow-up, 60 developed schizophrenia and 92 developed other non-affective psychoses. Poor scores for each of the five tests were associated with 3- to 14-fold increased risk of psychosis, particularly schizophrenia. Controlling for birth-related exposures, including birth weight, and parental education did not attenuate these associations. CONCLUSIONS Poor intellectual performance at 18 years of age is associated with early-onset psychotic disorder. Associations do not appear to be confounded by prenatal adversity or childhood circumstances, as indexed by parental education.