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 would upwardly bias the relationship between schizophrenia and bipolar disorder. However, when we analyzed sibling risks without co-morbidity there was still a highly significant increase in risk. Further, in the analyses using hierarchical diagnoses, estimates were almost exactly the same. Thus, we believe that the cumulative evidence suggests that there are common genetic etiologies between schizophrenia and bipolar disorder. Nonetheless, a considerable proportion of the genetic variance was not in common with the other disorder both for schizophrenia and bipolar disorder. Thus, there probably exist sets of genes associated with the risk for both diseases as well as sets of sets of genes associated with the risk unique for only one of the diseases, which should be considered in future research as well as clinical settings.
