Delusions and hallucinations do not happen just to psychotic people and may be completely normal, researchers say.

Hearing voices and seeing things that others cannot are not necessarily signs of psychosis and may be more common than people think, researchers say.

A study has found about five per cent of people at some point in their lives experience delusions and hallucinations - and they are reasonably well functioning.

Professor John McGrath said the study, led by the University of Queensland and Harvard Medical School, involved comprehensive mental health surveys of more than 31,000 people from 19 countries.

"We used to think that only people with psychosis heard voices or had delusions. But now we know that otherwise healthy, high-functioning people also report these experiences," he said.

"Of those who have these experiences, a third only have them once, and another third only have two to five episodes across their life. These people seem to function reasonably well."

Researchers found that auditory hallucinations were more common in women than men, and were also more common in people from wealthier countries.

Professor McGrath said the findings could lead to new research into the causes of isolated symptoms.

"In particular, we are interested in learning why some people recover, while others may progress to more serious disorders such as schizophrenia," he said.

"We need to rethink the link between hearing voices and mental health - it's more subtle than previously thought."