Objective: Impairment in social functioning is a central feature of schizophrenia and is known to be evident before the onset of psychosis, acting as a potential vulnerability marker. The aim of the present study was to test the hypothesis that social impairment is simultaneously a state and trait marker of risk for schizophrenia and schizophrenia-related disorder.

Method: Social functioning was examined in three groups: ultra-high-risk subjects (UHR, n =32), genetic high-risk subjects (GHR, n =32), and age- and IQ-matched healthy controls (HC, n =30). Social functioning was assessed using the Social Functioning Scale (SFS), and prodromal symptoms were assessed in high-risk subjects using the Comprehensive Assessment of At-Risk Mental States (CAARMS).

Results: Both the UHR and GHR groups exhibited significantly impaired social functioning compared with the HC group, and the UHR group was more impaired than the GHR group. In the UHR group, duration of prodromal symptoms was related to impaired 'interpersonal behaviour'. Positive and negative symptoms were not significantly associated with social functioning, whereas disorganized and general symptoms were significantly correlated with poor 'independence-competence' in UHR individuals.

Conclusion: The findings support the hypothesis that impairment in social functioning is both a trait and state marker of risk for schizophrenia and other psychotic disorders, implying that social impairment constitutes a mediating vulnerability indicator of psychotic disorders including schizophrenia.