People who have a brother or sister with schizophrenia are 10 times more likely to develop the mental illness, a new study out of Israel suggests.
Researchers also found increased risks for bipolar disorder when a sibling had been diagnosed with it.
Lead researcher Dr Mark Weiser, from the department of psychiatry at Sheba Medical Centre in Tel Aviv, called the results "quite striking".
"This is a large study which allows us to put meaningful figures on the risks of developing mental disorders after they have arisen in a brother or sister," he said.
However, the study only found an association between mental health disorders and risk to siblings; it did not prove cause and effect.
The study involved siblings of more than 6,000 Israeli patients who were diagnosed and hospitalised with schizophrenia, bipolar disorder or depression. They were compared to about 75,000 healthy peers from an Israeli population database.
Scientists already knew that people who have a close relative with a mental illness have higher odds of developing the same disorder. By comparing relative disease rates, the researchers behind the new study said they were able to determine the increased risk for siblings.
Among people who have a sibling with schizophrenia, the risk of developing a schizoaffective disorder increased six-fold to eight-fold. The risk of developing bipolar disorder was 20 times higher than for the general population, the study authors said.
In addition, siblings of people with bipolar disorder were four times more likely to develop bipolar disorder, schizophrenia and other psychiatric problems, the study found.
The findings were presented at the annual meeting of the European College of Neuropsychopharmacology (ECNP), in Vienna, Austria.
"These results are important clinically, as they encourage mental health workers to be aware of the increased risk of psychiatric disorders in siblings of patients," Weiser added in an ECNP news release. "These results can also be used by researchers studying the genetic underpinnings of mental disorders."
Studies presented at medical meetings are considered preliminary until published in peer-reviewed journals.