A family history of schizophrenia increases the risk that a person will develop the illness, but environmental influences like place and season of birth are also significant risk factors, according to the findings of a large epidemiological study by Danish researchers.

The study, published in today's issue of The New England Journal of Medicine, found that having a parent or a sibling with schizophrenia increased a person's chances of developing the illness by about sevenfold to ninefold. But people who are born in the winter months, in particular in February or March, or who are born in urban areas, are also slightly more likely to develop schizophrenia than people born at other times of the year or in rural regions.

People born in Copenhagen, Denmark's capital, were 2.4 times as likely to develop schizophrenia as those born in rural areas, the study found. February and March were the months of birth associated with the highest risk; August and September were associated with the lowest risk.

But the risk to the individual represented by time and place of birth is still very small, said Dr. Preben Bo Mortensen, the lead author of the study, who is a psychiatric epidemiologist at the Institute for Basic Psychiatric Research at Arhus University Hospital in Riskow.