Change in IQ in schizophrenia patients and their siblings: a controlled longitudinal study

Lower intelligence quotient (IQ) has frequently been reported in patients with schizophrenia. However, it is unclear whether IQ declines (further) after illness onset and what the familial contribution is to this change. Therefore, we investigate IQ changes during the course of illness in patients with non-affective psychosis, their siblings and controls.

Data are part of the longitudinal Genetic Risk and Outcome of Psychosis (GROUP) study in the Netherlands and Belgium. Participants underwent three measurements, each approximately 3 years apart. A total of 1022 patients with non-affective psychosis [illness duration: 4.34 (s.d. = 4.50) years], 977 of their siblings, and 565 controls had at least one measure of IQ (estimated from four subtests of the WAIS-III).

At baseline, IQ was significantly lower in patients (IQ = 97.8) and siblings (IQ = 108.2; p < 0.0001) than in controls (IQ = 113.0; p < 0.0001), and in patients as compared with siblings (p < 0.0001). Over time, IQ increased in all groups. In siblings, improvement in IQ was significantly more pronounced (+0.7 points/year) than in patients (+0.5 points/year; p < 0.0001) and controls (+0.3 points/year; p < 0.0001). IQ increase was not significantly correlated with improvement in (sub)clinical outcome in any of the groups.

During the first 10 years of the illness, IQ increases to a similar (and subtle) extent in a relatively high-functioning group of schizophrenia patients and controls, despite the lower IQ in patients at baseline. In addition, the siblings' IQ was intermediate at baseline, but over time the increase in IQ was more pronounced.