In analyzing three decades of generational information on 2 million families in Sweden, researchers from the Karolinska Institute in Stockholm found that blood relatives of people with either mental disorder had a notably higher risk of developing bipolar disorder or schizophrenia during their lives, compared to the general population.

According to the study, published in this week's edition of The Lancet, brothers or sisters of people with these conditions were nine times more likely to be schizophrenic and had eight times the risk of developing bipolar disorder.

The odds were less steep but still high for half-siblings. Brothers and sisters with the same mother had a 3.6 times greater chance of having schizophrenia and a 4.5 times greater risk of bipolar disorder. Paternal half-siblings were roughly 2.5 times more likely to have either condition.

Increased risk of schizophrenia was also found in relatives of people with bipolar disorder. This included adopted children if their biological parents had bipolar disorder.

"Similar to molecular genetic studies, we showed evidence that schizophrenia and bipolar disorder partly share a common genetic cause. These results challenge the current nosological (disease classification) dichotomy between schizophrenia and bipolar disorder, and are consistent with a reappraisal of these disorders as distinct diagnostic entities," the authors wrote.

More information

The National Alliance for Research on Schizophrenia and Depression has more about schizophrenia and bipolar disorder .