Theory of mind impairment in schizophrenia: Meta-analysis

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Abstract

There is now substantial evidence for Theory of mind (ToM) impairment in schizophrenia. Despite this, we know little about how dynamic (state) variables and broad clinical, cognitive and medication characteristics moderate the precise magnitude of the observed ToM deficit during task performance. Meta-analyses were conducted using 36 studies that reported continuous data regarding ToM performances of schizophrenia patients and healthy control subjects. These 36 studies included 1,181 (67% male) patients with schizophrenia and 936 (58.3% male) healthy control subjects. Individual analyses were also conducted for the Hinting and the Eyes tasks. The effects of moderator variables were studied by both subgroup and meta-regression analyses. The effect sizes (Cohen's d) for overall ToM performance and the individual tasks were large (d = 0.90–1.08). In “remitted” patients, the degree of ToM impairment was less pronounced than non-remitted patients (d = 1.21) but it was still significant (d = 0.80). Moreover, the distribution of effect sizes was more homogeneous for the individual tasks, especially in “remitted” patients. General intellectual deficits observed in schizophrenia patients contributed to their ToM impairment only in the remission phase of the illness. While state variables and task specific differences explain a large degree of the heterogeneity of the ToM findings observed in previous studies, the persistence of ToM deficits in “remitted” patients suggests there are trait related mentalising impairments in schizophrenia. Our review also suggests that future research should consider the potential moderating influence of IQ deficits on ToM performance in “remitted” patients, as well as the potential effects of residual symptoms.

Introduction

Theory of mind (ToM) deficits in schizophrenia have been investigated by a substantial number of studies since Frith (1992) proposed a model suggesting a relationship between mentalising (ToM) impairment and specific symptoms of schizophrenia. Two systematic reviews (Brüne, 2005, Harrington et al., 2005a, Harrington et al., 2005b) and a recent meta-analysis (Sprong et al., 2007) showed that nearly all published empirical studies reported ToM impairment. Sprong et al. (2007) reported a large overall effect size (d = 1.25) for 29 published studies. Interestingly, the magnitude of the impairment was attenuated but remained significant in remitted patients (d = 0.69). While this result suggests that acute psychosis has a significant moderating influence on ToM performance, it also implies that mentalising deficits might be trait-characteristics of schizophrenia.

While there is no doubt that schizophrenia patients perform poorly on ToM tasks, there are important issues that were not adequately considered in previous studies and reviews. The main issues relate to (i) heterogeneity of ToM tasks and their neurocognitive demands; (ii) psychometric properties of ToM tests; and (iii) the influence of clinical and demographic characteristics on ToM performance. First, while the study of Sprong et al. (2007) provided the first attempt to qualitatively analyze the available data, it did not adequately take the heterogeneity of ToM tasks into account. Previous studies have used different ToM tests comprised of very different test characteristics. For example, different tasks rely on different neurocognitive processes and deficits of schizophrenia patients in working memory, executive functions and attention, which could differentially influence (and interact with) patients` performance on various ToM tasks. Secondly, ToM tasks are not well standardized and their psychometric properties have not been studied. For example, while some of the false belief tasks included only single or a few stories, others included more stories. In support of this, Sprong et al. (2007) reported heterogeneity for false belief tasks that may be related to variability of these tasks. Finally, the moderating influence of clinical and demographic characteristics on ToM dysfunction in schizophrenia may be underestimated and confound interpretations. For example, general intellectual deficits of the patients with schizophrenia could affect performance of mentalising tasks.

In this context, meta-analysis is a useful tool for systematically combining all research in this area to identify the most robust aspects of ToM impairment in schizophrenia and to understand how performance on such tasks is moderated by state and trait factors. In this way, we may be better able to understand the influence of moderating influences on social cognition disturbances that characterize schizophrenia. Our aims were to: (i) to conduct meta-analyses for individual or more homogeneous tasks that can estimate a reliable overall effect size for the different aspects of ToM impairment; and (ii) to study the effects of potentially confounding factors related to clinical and demographic characteristics on ToM impairment in schizophrenia.

Section snippets

Study selection

A literature search was conducted using the databases Pubmed, Medline, EMBASE, SCOPUS and PsycINFO to identify the relevant studies (January 1990 to May 2008). The following keywords were used: schizophrenia, psychosis, Theory of Mind, mentalising and social cognition. The early online editions of the major psychiatric journals were also checked and reference lists of published reviews and studies were used to identify additional relevant publications.

Inclusion criteria

  • 1)

    Studies should be published in

Results

Our extensive search strategy identified 73 studies that investigated TOM impairment in schizophrenia. 17 of these studies did not include a healthy control group. Another 20 were excluded since they did not meet the inclusion criteria of the study (dichotomous variables, nonincluded tasks, sample overlap or insufficient data). The final sample consisted of 36 studies (Table 1). These 36 studies included 1181 (67% male) patients with schizophrenia and 936 (58.3% male) healthy control subjects.

Discussion

Schizophrenia patients had mentalising impairments with large effect sizes observed for all tasks. The distribution of effect sizes was more homogeneous for individual tasks of ToM, especially in “remitted” patients. One of the main findings of this study was that ToM impairment was strongly influenced by acute psychosis. Overall sample had prominent impairment on all ToM tasks while “remitted” patients had reduced but still significant, ToM impairment. Contrasting with the earlier views, this

Role of funding source

None.

Contributors

Emre Bora designed the study, collected and analysed the data and wrote the first draft of the manuscript. Murat Yucel and Christos Pantelis contributed to design of the meta-analysis. All authors contributed to and approved the final draft of the paper.

Conflict of interest

All authors report no conflict of interest.

Acknowledgement

Murat Yücel was supported by a National Health & Medical Research Council (NH&MRC) Clinical Career Development Award (I.D. 509345). NHMRC.

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