BACKGROUND: There appear to be two kinds of delusion of reference, which vary independently: delusions of observation and delusions of communication. It has been suggested that delusions of communication might derive from an impairment in reality discrimination, though the impairment would be centred on non-verbal channels in delusions of communication as opposed to verbal channels in auditory hallucinations. METHOD: Patients (N=64) with acute psychotic symptoms were recruited according to a 2 x 2 design: presence versus absence of delusions of communication crossed with presence versus absence of auditory hallucinations. They were presented with 100 brief video clips in which an actor either made a well-known gesture or an incidental movement, with the clips being obscured by visual noise. For each clip, the patients indicated how confident they were that a gesture was portrayed. RESULTS: According to a signal detection analysis, all groups showed adequate sensitivity and the groups did not differ in sensitivity, but patients with delusions of communication showed a bias to report gestures which was not shown by patients with hallucinations. A control group of healthy volunteers (N=57) showed significantly greater sensitivity than the patients and a more conservative bias than patients with delusions of communication. CONCLUSIONS: A bias to report gestures is not part of a general tendency to externalize one's own thoughts but may be the result of a reality discrimination deficit that is specific to delusions of communication. A possible theoretical explanation for such a deficit is discussed.