Elaboration on Premorbid Intellectual Performance in Schizophrenia

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Abstract
Evidence exists to indicate that, as a group, individuals who later will be hospitalized for schizophrenia have intellectual deficits that predate the manifestation of psychotic symptoms.1-4 However, there are inconsistent findings and divergent hypotheses as to the onset and course of the premorbid intellectual impairment. A majority of studies5-7 indicate that intellectual deviations from norms are already present during childhood, and that the gap between individuals with future diagnoses of schizophrenia and normal individuals does not widen during this period (for a different view, see Kremen et al8). However, several lines of evidence from large retrospective studies,9 studies of clinical samples,10 studies of high-risk individuals,11,12 and family and twin studies13,14 suggest that the deviations from intellectual norms might increase as the future patient moves from childhood into adolescence and approaches the onset of active psychotic illness. Only 2 population-based studies addressed this question. In a longitudinal study of the 1946 British birth cohort, Jones et al2 found that the difference in intellectual performance between future patients and controls widens between ages 8 to 15 years. In contrast, no increase in the magnitude of impairment in future patients was found in the 1958 British birth cohort.15

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