In this population-based study of over 2 million Swedish families, we found evidence of a substantial genetic relationship between schizophrenia and bipolar disorder. All classes of biological relatives of probands with bipolar disorder had a significantly increased risk for schizophrenia, and the genetic correlation (i.e., the correlation between the genetic effects influencing the liabilities for the two disorders) was .60. Further, adopted children whose biological parents had one of the disorders had significant increase in risks for the other disorder. These results agree with previous molecular genetic studies1, as well as twin15 and family17 data in suggesting a common genetic influences in schizophrenia and bipolar disorder, even though it is at odds with some family studies which have used clinicians to review the personal interview and hospital and out-patient records from the course of illness3,4. It is well-known that patients with one diagnosis sometimes evolve into the other. If this development of psychotic diseases is due to misclassification at the first diagnoses rather than to the fact that the individual has had both disorders, our approach of using non-hierarchical diagnoses
