Schizophrenia and manic depression could have similar genetic causes, researchers suggest.
The researchers looked at the brains of people with the conditions
The flaw appears to lie in genes which affect how the central nervous system develops.
Researchers from the University of Cambridge say the findings are surprising because the conditions are so different.
Schizophrenia and manic depression, or bipolar disorder, affect around 2% of the population.
The researchers looked at the brains of 15 people who had had schizophrenia, 15 who had had bipolar disorder, and 15 who had been healthy.
They looked at genes associated with the formation of the myelin sheath which covers and protects nerves and enables the efficient conduction of electrical impulses through the nervous system.
The genes were either linked to the development of oligodendrocyte cells, which make up the sheath tissue, or the development of the myelin itself.
The team looked at mRNA, which carries the genetic code, to look at how effectively the genes were expressed in the different groups.
They found these genes were less active in people with schizophrenia and bipolar disorder, and there were also faults in how the remaining genes were expressed.
Similar changes were seen in the brains of people with both conditions.
Dr Sabine Bahn, who led the research, published in The Lancet, said: "We believe that our results provide strong evidence for oligodendrocyte and myelin dysfunction in schizophrenia and bipolar disorder.
"The high degree of correlation between the expression changes in schizophrenia and bipolar disorder provide compelling evidence for common pathophysiological pathways that may govern the disease phenotypes of schizophrenia and bipolar disorder."
Writing in the journal, Dr Kenneth Davis of the Mount Sinai School of Medicine, New York, USA, said: "The observation that at least some myelin-related gene- expression deficits are common between individuals with schizophrenia and bipolar disorder is intriguing because schizophrenia and bipolar disorder have different symptom profiles and require treatment based on quite different neurotransmitter systems."
A spokesperson for Rethink, the charity for people with schizophrenia, said: "We welcome any new research or progress into understanding the causes of schizophrenia, but it would need to be checked before it would make a difference to the thousands of people living with severe mental illness in the UK.
"In the meantime, reaching people early with the right care and treatment is the best way of recovering a meaningful and fulfilling life."