People from broken homes may be more prone to psychotic illnesses such as schizophrenia, research suggests.
Psychosis may be linked to social adversity
Researchers said their findings suggest the illnesses are not simply brain diseases, but linked to factors such as social adversity.
They found much higher rates among black people, who were also more likely to come from broken homes.
The study, by London's Institute of Psychiatry, will appear in the journal Psychological Medicine.
The researchers examined data on people in south east London, Bristol and Nottingham, including 780 who showed signs of a psychotic illness.
They found schizophrenia was nine times more common in people from African Caribbean origin, and six times more common in people from black African origin than in the white British population.
In a second paper, they found that separation from one or both parents for more than a year before the age of 16, as a consequence of family breakdown, was associated with a 2.5 fold increased risk of developing psychosis in adulthood.
Family breakdown of this type was found to be more common in the African-Caribbean community (31%) than the white community (18%).
Researcher Dr Craig Morgan said: "These findings provide evidence that early social adversity may increase the risk of later psychosis.
"Such early adversity may be one factor contributing to the high rate of psychosis in the African-Caribbean population."
More work needed
However, Dr Morgan said more work was needed to fully understand how specific types of early social adversity might interact with pyschological and biological factors to cause psychosis.
Professor Robin Murray, who also worked on the research, said: "For the last 30 years the traditional view has been that psychosis is largely a genetic brain disease, and most psychiatrists have thrown out the view that social factors can have a major impact.
"These findings suggest it is not just a brain disease, and that social factors can also contribute to the onset of illness."
Professor Murray dismissed the idea that drug taking might contribute to raised rates of psychosis among the black population.
He said evidence showed that drug taking was no higher among black people than the general population.
He said it was possible that the discrimination and disruption encountered by migrants to the UK might play a role in their increased vulnerability to psychosis.
Paul Corry, of the mental health charity Rethink, said there was evidence to suggest that although psychotic illness was linked to the genes, it often took an external trigger for symptoms to become apparent.
He said: "These findings underline the need to approach the treatment of schizophrenia not just in purely medical, drug-based terms, but also by taking into account the wider social context that the patient is inhabiting at the time, and trying to ensure they are offered relevant support."
It is thought that around 1% of the population develop schizophrenia, or related conditions, such as manic psychosis, and depressive psychosis.
Up to 300,000 people have been diagnosed with schizophrenia in the UK.
All the conditions are associated with hallucinations, delusions and bizarre forms of behaviour.
Pyschotic illnesses have been linked to raised levels of the mood-altering chemical dopamine in the brain.
The Institute of Psychiatry is based at King's College London.