Schizophrenia, theory of mind, and persecutory delusions

Introduction. There is already a substantial body of evidence supporting Frith's (1992) theory that theory of mind (ToM) is impaired in people with schizophrenia. However, a specific relationship between impaired ToM and paranoid delusions, while intuitively reasonable, has only been demonstrated in two studies to date. Correspondence should be addressed to Richard J. Siegert, Rehabilitation Teaching and Research Unit, Department of Medicine, Wellington School of Medicine e?mail: rsiegert@wnmeds.ac.nz This research was supported by a Victoria University Science Faculty research grant. Thanks to Max Coltheart and other members of the Macquarie Centre for Cognitive Science who provided valuable feedback and support in the early stages of this research. Preliminary results were presented at the Australian Conference of Cognitive Neuropsychology and Neuropsychiatry, Deakin University, 2001. Methods. A total of 25 participants with schizophrenia were classified as paranoid or nonparanoid and compared with 38 healthy controls on a variety of ToM tasks. These tasks included verbal and nonverbal, and first and second order ToM tasks. Results. Participants with schizophrenia performed significantly more poorly than healthy controls on both the first and second order verbal ToM tasks but not on the nonverbal ToM tasks. However, the ToM deficit was only observed for those participants with schizophrenia who had persecutory delusions. There was also a strong relationship observed between the severity of persecutory delusions and length of illness. Conclusions. This study represents only the third demonstration of a specific link between paranoid delusions and ToM impairment. Reasons why previous findings on this issue have been so inconsistent are considered. Further research is needed to explore the relationships among paranoia, ToM, and length of illness.