IQ decline in cross-sectional studies of schizophrenia: methodology and interpretation.

Some researchers have compared neuropsychological performance in schizophrenia groups with and without presumed IQ decline. Inherent in this approach is an assumption that group differences are due to different IQ trajectories (stable vs. declining), but neuropsychological differences could be a function of current IQ regardless of the presence or absence of previous IQ decline. We examined this issue in 93 normal controls and in 80 patients classified as having preserved (27.5%), deteriorated (50%), or compromised (22.5%) intellect based on IQ and reading recognition-IQ difference scores. We also examined group differences in verbal and performance IQ. Deteriorated patients had the largest verbal performance-IQ differences. They were more neuropsychologically impaired than the preserved group (average effect size=0.43), but deteriorated patients also had significantly lower current IQs. When subgroups of preserved and deteriorated patients with equivalent current IQs were compared, neuropsychological differences were essentially eliminated (average effect size=0.10); however, both groups were significantly more impaired than controls with similar IQs. Neuropsychological impairment, even in patients with apparently preserved IQ, is consistent with a prefrontal-dysexecutive syndrome. Overall, these results strongly suggest that differences in current neuropsychological function in schizophrenia are attributable primarily to current IQ instead of to IQ trajectory over time.