Low IQ scores in schizophrenia: primary or secondary deficit?

Schizophrenia is consistently associated with lower IQ compared to the IQ of control groups, or estimated premorbid IQ. It is not likely that the IQ scores deteriorate during the prodromal phase or first psychotic episode; they are already present before the onset of the prodromal phase and have been detected in childhood.

We investigated cognitive functioning and IQ levels in a group of 36 patients with schizophrenia or other psychotic disorders.

The IQ scores in our sample were lower than average. The IQ showed a relation with attention, memory, speed of information processing and some aspects of executive functioning. However, when IQ scores were corrected for processing speed, they were no longer below average.

These findings are important in considering the value of intelligence levels in schizophrenia. IQ scores should be judged in combination with cognitive functioning and school career to assess a patients capabilities in society. Cognitive functions and other variables might have a considerable influence on IQ scores. This rises the question of whether the low IQ scores are a primary or secondary deficit. Schizophrenia patients may have normal IQs, but could be less capable of making an IQ-test.