Harvard Mental Health Letter

In Brief: Schizophrenia and bipolar disorder may share genetic origins

A long-running debate in psychiatry is whether schizophrenia and bipolar disorder are really two distinct illnesses, or instead represent different manifestations of a single mental illness. A large study provides evidence that, however they are classified, schizophrenia and bipolar disorder are more alike genetically than they are different.

Investigators at the Karolinska Institute examined two national registry databases in Sweden, which included information about family psychiatric histories of 9 million people from 1973 to 2004. They identified 35,985 individuals with hospital discharge diagnoses of schizophrenia and 40,487 with bipolar disorder. The researchers excluded patients with schizoaffective disorder, a separate condition that shares symptoms of both illnesses.

The researchers then determined the risk for schizophrenia and bipolar disorder, depending on the degree that genes and environmental factors were shared, by looking at relationships between parents, siblings, half-siblings, and adoptees from families with either diagnosis.

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