"Schizophrenia is the price that homo sapiens pay for language."
That is the controversial theory of one leading psychiatrist.
Professor Tim Crow believes that the difference in the development of the human brain from the primate brain - which allows us to process thought and speech - is linked to why psychotic illnesses occur.
The human brain has developed to have a strong regional bias, so each side of the brain performs certain roles - for example, speech is controlled by the left side of the brain.
Professor Crow of the mental health charity Sane's Prince of Wales International Centre in Oxford, suggests the division boundaries between certain areas of the brain, particularly those which are concerned with language and thought, are "blurred" in people with psychoses.
"It would be wrong to look for a single root cause"
People with these conditions may hear their inner thoughts as external voices, or believe thoughts have been inserted in their head, suggesting the normal divisions do not exist.
The reason for this, he says, is that their brains do not have the bias, or asymmetry, seen in healthy people.
Brain asymmetry means that areas control certain things, so the left-hand side controls language.
He said: "Asymmetry appears to be less pronounced in people with psychoses."
Understanding 'key'
The genetic reasons for these differences are hundreds of thousands of years old, he says.
There are particular areas on the sex chromosomes of humans which differ from those seen in primates.
Professor Crow suggests there is an "asymmetry gene" on the sex chromosomes, that gives human brains the capacity for language.
He suggests that variation in an "asymmetry gene" in one of these areas could be the factor which determines if someone is going to develop schizophrenia.
The search for this key gene is still ongoing, but Professor Crow believes it could be the answer to the explanation for the continuing presence of schizophrenia in societies around the world.
He said it was important to understand what caused psychotic illnesses.
"You can't do anything if you haven't got the understanding. If you have, then you might be able to do something. It's possible you might be able to develop treatments."
Paul Corry, director of campaigns and communication for the mental health charity Rethink said: "We really don't understand the origins of schizophrenia or how it is caused today.
"However, it would be wrong to look for a single root cause. There may be a genetic vulnerability in some people that can be triggered by environmental and emotional factors such as bereavement, moving home or a breakdown in relationships.
"Yet there is no obvious genetic link in other people.
"What we need to understand is how this complex of factors interacts to create a condition in one person that doesn't appear in another.
"In the meantime, getting the right medical help and social support at the right time still gives people the best chance of recovery."
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