Purpose: To investigate familial clustering of schizophrenia, bipolar disorder, and major depressive disorder.
Methods: Combining data from a psychiatric case registry and Statistics Netherlands provided information on 4,673 affected probands and 18,692 matched population controls.
Results: Probands with schizophrenia had relative risks (RRs) for having a sibling with schizophrenia of 3.77 (95% confidence interval (CI): 2.60-5.46) and with bipolar disorder of 1.79 (95% CI: 0.64-4.96) as compared with a reference proband. Probands affected with bipolar disorder have an RR of 6.51 (95% CI: 2.60-16.29) for having a sibling with bipolar disorder and of 1.71 (95% CI: 0.71-4.14) for having a sibling with schizophrenia as compared with a reference proband. Probands affected with major depressive disorder also have increased risk for having a sibling with schizophrenia (RR: 2.04, 95% CI: 1.54-2.72) as compared with a reference proband, which was similar to the risk for having a sibling with major depressive disorder (RR: 1.91, 95% CI: 1.63-2.24) or bipolar disorder (RR: 2.06, 95% CI: 1.18-3.60).
Conclusion: Our findings suggest, as previous studies have, that risk across schizophrenia and bipolar disorder is considerably lower (twofold) than within diagnostic entities, whereas for major depressive disorder risk is similar within and across diagnostic entities.