Theory of mind in schizophrenia

Mentalising impairment (an impaired ability to think about people in terms of their mental states) has frequently been associated with schizophrenia.

To assess the magnitude of the deficit and analyse associated factors.

Method
Twenty-nine studies of mentalising in schizophrenia (combined n=1518), published between January 1993 and May 2006, were included to estimate overall effect size. Study descriptors predicted to influence effect size were analysed using weighted regression-analysis techniques. Separate analyses were performed for symptom subgroups and task types.

Results
The estimated overall effect size was large and statistically significant (d=–1.255, P<0.0001) and was not significantly affected by sample characteristics. All symptom subgroups showed significant mentalising impairment, but participants with symptoms of disorganisation were significantly more impaired than the other subgroups (P<0.01).

Conclusions
This meta-analysis showed significant and stable mentalising impairment in schizophrenia. The finding that patients in remission are also impaired favours the notion that mentalising impairment represents a possible trait marker of schizophrenia.

INTRODUCTION

'Theory of mind' and 'mentalising' refer to the cognitive ability to attribute mental states such as thoughts, beliefs and intentions to people, allowing an individual to explain, manipulate and predict behaviour. In 1992 Frith proposed a relationship between theory of mind and schizophrenia, and argued that several symptoms of schizophrenia could be explained by mentalising impairment (Frith, 1992). This led to a substantial body of research which has recently been critically reviewed twice (Brüne, 2005a; Harrington et al, 2005a). In both reviews it was concluded that theory of mind is impaired in individuals with schizophrenia. Although these reviews were executed thoroughly, they are limited to a qualitative description of the observed deficit, thus lacking important information on the magnitude of the effect. The purpose of this meta-analysis is to produce a synthesised effect size estimate that has considerably more power than the individual studies. In addition, effects of study characteristics on the findings are analysed.

DISCUSSION

The aim of this meta-analysis was to investigate the extent of mentalising impairment in people with schizophrenia. By combining 29 studies, a total sample size was created of over 1500 participants. The overall effect size was –1.1255, indicating that on average the theory of mind performance of participants with schizophrenia is more than one standard deviation below that of healthy controls. According to a widely used convention for appraising the magnitude of effect sizes this is considered a large effect (Cohen, 1988). Homogeneity analysis showed that the mean effect size of the combined samples is a good estimate of the typical effect size in the population. The large fail-safe number makes the `file drawer' problem, which is a limitation of some meta-analyses, negligible.