These experiments suggest that auditory hallucinations are the result of the mind failing to brand its actions as its own. Watching what the brain does during these hallucinations may clarify how that works, and what differences in the brain create these experiences.

“When your brain signals to generate a movement,” Shinn, the psychiatrist at Harvard, told me, “there is a parallel signal [known as an efference copy] that basically says ‘this is mine, it’s not coming from outside.’” This helps creates the sense of where a person is in space, that their hand belongs to them and it is moving from point A to B. In this way, the body labels its motions, and a possible parallel may exist for speech and thought. When people hear voices, they may be hearing ‘unmarked’ thoughts they do not recognize as their own.