In his 2003 book, “Madness Explained,” Bentall draws on the theory that auditory hallucinations may have their roots in what psychologists call “inner speech.” All of us, every day, produce a steady stream of silent, inward-directed speech: plans, thoughts, quotations, memories. People hear voices, Bentall argues, when they make faulty judgments about whether this inner speech is the product of their own consciousness or of something alien to their consciousness. Lapses in what researchers call “source monitoring” may occur for a number of reasons — because an individual is primed to expect a perception to occur, because the level of background noise makes it difficult to separate what is internal from what is external, because he or she is in a state of emotional arousal.

This theory raises the critical question of why making source-monitoring errors ... results in psychosis: why, when people mistake their private speech for someone else’s, does it cause them to grow so distressed

a fundamental difference between voice-hearers in the community and voice-hearers under psychiatric care is that the latter think negatively about their experience.