Reasoning about physical causality and other's intentions in schizophrenia.

Introduction: Several studies have shown that schizophrenic patients have impaired theory-of-mind skills involving the attribution of false beliefs and intentions to others. Despite the methodological difficulties of false belief protocols, experiments have concluded that the deficit is specific and cannot be explained in terms of a general cognitive impairment. The situation, however, remains unclear as far as the attribution of intentions to others is concerned. We propose the use of nonverbal control tasks based on physical causality to demonstrate the specificity of the impairment of attribution of intentions.

Methods: We compared the performances of schizophrenic patients and normal subjects on the attribution of intentions and on two control tasks involving physical causality (reasoning about the weight, location, speed, and the physical properties of objects). 25 schizophrenic patients and 25 healthy subjects participated in the study. Each subject was assessed under three conditions using comic strips: attribution of intentions, physical causality with characters, physical causality without characters.

Results: The performances of schizophrenic patients differed significantly from those of healthy subjects only in the attribution of intentions condition. This difference was also found when only subjects having optimal performances in physical logic were considered and when verbal IQ was considered as a co-variable.

Conclusions: Nonverbal attribution of intentions to others is specifically impaired in schizophrenia. This conclusion is in agreement with recent neuroimaging findings.