People are 10 times more likely to develop schizophrenia if they have a sibling with the condition, a new study has found.

While it is widely acknowledged that people have an increased risk of developing mental health conditions if their siblings are affected, until now, the risks have not been quantified.

However, international researchers have managed to do this using data on patients hospitalised in psychiatric wards in Israel and their siblings. This involved comparing over 6,100 people who had been hospitalised with schizophrenia, bipolar disorder or depression, with more than 74,000 controls.

They found that people who had a brother or sister with schizophrenia were 10 times more likely to develop the condition themselves. They were also six to 10 times more likely to develop schizoaffective disorder, which occurs when a person experiences a combination of schizophrenia symptoms, such as hallucinations, and mood disorder symptoms, such as mania or depression.

Meanwhile, people who had a brother or sister with bipolar disorder had a seven to 20 times increased risk of developing the condition compared to the general population.

"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. The figures are quite striking, with x10 risk of developing schizophrenia, and similar risks once a sibling has developed bipolar disorder," commented the study's lead researcher, Prof Mark Weiser, of Tel Aviv University.

The researchers believe these findings are important because they may encourage healthcare professionals working in this area to be more aware of the risk of mental health conditions in the siblings of patients.

"These results can also be used by researchers studying the genetic underpinnings of mental disorders, as they indicate that the same genes might be associated with increased risk for various psychiatric disorders," Prof Weiser added.

Details of these findings were presented at the recent annual European College of Neuropsychopharmacology (ECNP) Congress in Vienna.