Patients with psychosis struggle with scalar implicatures

Pragmatic language difficulties in people with psychosis have been demonstrated repeatedly but one of the most studied types of pragmatic language, i.e. scalar implicatures (SIs), has not yet been examined in this population. SIs are a special kind of pragmatic inferences, based on linguistic expressions like some, or, must. Such expressions are part of a scale of informativeness organized by informativity (e.g. some/many/all). Although semantically the less informative expressions imply the more informative ones, pragmatically people generally infer that the use of a less informative expression implies that the more informative option is not applicable. Based on the pragmatic language difficulties of people with psychosis we hypothesized that they may be less likely to derive these pragmatic SIs. We conducted two studies in which the ability of people with psychosis to derive SIs was compared to that of healthy controls matched for age and educational level. In the second study we additionally explored the possible link between the capacity to derive SIs and theory of mind (ToM) ability. In general, people with psychosis were less likely to derive SIs than controls. However, the patient group was not homogeneous: half had problems deriving SIs, the other half did not. This dichotomization seems linked to ToM ability because in the patient group, better ToM was associated with a higher ability to derive SIs. Based on the nature of the stimuli used in the SI-task we speculate that this link may not be a direct but an indirect one.