New evidence confirms a long-held theory that people with schizophrenia hear 'voices' in their heads by misattributing inner speech as external.

"This idea's been around for 50 years, but it's been very difficult to test because inner speech is inherently private," says Thomas Whitford, a psychology researcher at the University of New South Wales.

Using EEG (electroencephalography) to measure brainwaves, Whitford and his colleagues tested the way that the brain reacts to inner speech, and, in people with schizophrenia, the way their brains react to auditory hallucinations.

"When we speak – even just in our heads – the part of the brain that processes sounds from the outside world becomes less active," Whitford explains. "This is because the brain predicts the sound of our own voice. But in people who hear voices, this prediction seems to go wrong, and the brain reacts as if the voice is coming from someone else."