The reason the psychosis is so common goes to the root of what it means to be human, says Roger Highfield,

Scientists have found evidence for why schizophrenia may be so common, in research that suggests it is linked to the evolution of human attributes, such as creativity.

They have been baffled why the disorder affects one person in every 100 across all cultures, suggesting that it is linked with an important aspect of being human, yet causes such devastating effects that would suggest it would make a sufferer much less likely to survive and to pass on their genes.

Evidence has now been published that the evolution of a number of genes in the brain of Stone Age man may have provided humanity with the intellectual boost it needed to conquer the world, and outcompete rivals such as the Neanderthals, but left mankind vulnerable to the common psychosis.

Contrary to popular notion, schizophrenia does not involve a "split personality" but does involve delusions, in which patients may think others are plotting against them; hallucinations, where they hear voices or see figures that aren't real; and disorganised thinking and behaviour, where they may have trouble conversing or focusing on a task.

Data that suggests schizophrenia may be a side effect of a surge in evolution as humans adapted over hundreds of thousands of years to their environment - linked with creativity, language and artistic skills, problem solving and our ability to work together - are published in the Proceedings of the Royal Society Biological Sciences.

That is not to say that a genius has to be schizophrenic but that they contain a spectrum of genes that brings them closer to the psychosis, though the effects may be most apparent in a relative.

Researchers analysed the molecular evolution of 76 genes linked with the disorder and found that such schizophrenia genes, including some with the strongest links to the disorder such as DISC1, DTNBP1 and NRG1, were more likely to bear genetic variants that were favoured by evolution, than were a comparable control set of non-associated genes, functioning in similar processes in nerves.

"The world-wide presence of this disorder at an appreciable frequency, despite its impact on human health and reproductive fitness, is somewhat of a paradox," said Dr Steve Dorus from the University of Bath, who worked with Dr Bernard Crespi from Simon Fraser University, Canada, and Dr Kyle Summers from East Carolina University on the research.

"This may be explained by the existing theory that the condition represents, in part, a by-product of adaptive changes during human evolution. Our finding that positive evolutionary processes have impacted some genes underlying the disorder is consistent with this idea."

Schizophrenia has long been associated with creativity throughout recorded history, and it has been suggested it has been a side effect of the adaptive expansion of human abilities. This new study provides genetic evidence consistent with this idea. However, Dr Dorus said the genetic basis of creativity is unknown and its link to schizophrenia remains unproven.

"People with schizophrenia think in ways that are unexpected and surprising which may explain the link to creativity. But the problem with this theory is that the creation of culturally useful products - as opposed to just nonsense - also requires deployment of analytical and critical judgement, plus viable persistence in the face of opposition, not just novel ideas," commented Prof Raj Persaud, Consultant Psychiatrist at the Bethlem Royal and Maudsley Hospitals and Gresham Professor for Public Understanding of Psychiatry.

"So it's probably more likely that we all in fact exist along a spectrum of schizophrenia and creativity depending on the particular pattern of genes we inherit - this explains the high rates of creativity and schizophrenia found in the same families rather than so much in the same person.

"For example it is said that James Joyce had a daughter with schizophrenia and Bertrand Russell's family has several members with schizophrenia while some say Einstein had a son with the condition.

"There is a long standing argument (attributable primarily to Tim Crow, Oxford), that schizophrenia is the evolutionary 'price' paid for human speciation," added Prof David Porteous of the University of Edinburgh.

"As for the link between schizophrenia and creativity, this is not widely claimed, because of the seriousness of the condition. However, it is quite clear that for Bipolar Disorder there is a link (more poets, artists and writers than expected by chance), but it is a small sub-set of those affected. For the majority, like schizophrenia, bipolar is a disabling rather than enabling condition."

Schizophrenia is an umbrella term for a complex group of mental disorders. The debilitating mental illness has affected around 500,000 people in Britain alone, exacting a heavy toll on both patients and their families.

The research was funded in part by the Natural Sciences and Engineering Research Council of Canada, East Carolina University and the National Institutes of Health.