The use of a non-hierarchical definition of syndromes might be questioned. We therefore investigated the sibling risk where both probands and siblings with co-morbid schizophrenia and bipolar disorder were excluded. The risk for bipolar disorder in siblings of probands with schizophrenia were still significantly increased (Relative risk = 2.7; 95% confidence interval 2.3–3.1). Further, we also performed the model-fitting analyses using hierarchical diagnoses, where individuals with ≥2 schizophrenia diagnoses were classified as schizophrenic (and not bipolar disorder even if they have had 2 or more such diagnoses). As can be seen in the lower panel of Table 3, estimates were almost exactly the same, except for that nonshared environmental effects did not contribute to co-morbidity (because an individual could not be diagnosed with both disorders).
