For some people, hearing voices in their heads is a positive experience, not a sign of mental illness or cause for distress.
Researchers at the University of Manchester in England are aiming to find out why.
But studies by Dutch researchers that began in the 1990s found that some healthy people also regularly hear voices.
The scientists ran a program on Dutch television asking for volunteers who heard voices, and they got a surprising response.
Many of the people who contacted them did not find the voices disruptive, and had never felt the need to consult mental health services. Some even said they found the experience to be positive or inspirational.
The resulting studies found that more people might hear voices than psychologists had thought, perhaps around 4 percent of the population.
Aylish Campbell, a psychologist at the University of Manchester, is hoping to expand on the Dutch study by investigating why peoples' reactions to hearing voices vary so widely.
Campbell has just begun looking for study participants in Britain.
"We're looking for people who hear voices who have a range of experiences," she said.
Campbell and her colleagues suspect the variation could be caused by different life experiences.
Childhood traumas, beliefs that other people are untrustworthy or dangerous and feelings of vulnerability might cause people who hear voices to react with fear.
The experience might be enjoyed by people who have positive outlooks, they figure.
Campbell thinks anyone can hear voices, particularly when stressed. For example, those who are grieving over the recent loss of a loved one might hear that person's voice.
"It might just be a normal human experience," she said. "People are susceptible to different degrees."
Campbell hopes that learning what triggers different reactions could help develop new psychological therapies to help people — at least those who don't like the phenomenon — to cope with the voices.
"If we can understand a bit more about the factors, we might be able to use that knowledge to help people who do find it distressing," Campbell said.
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