Epidemiological comparison of schizophrenia and bipolar disorder.

Schizophrenia and bipolar disorder share many epidemiological features in common, but there are also differences. Both disorders share some risk factors, suggesting etiological antecedents that date to the perinatal period; these include excess of winter-spring births, abnormal dermatoglyphics, and probably an excess of perinatal complications. By contrast, an excess of urban births and an excess of minor physical anomalies are present in schizophrenia, but apparently not in bipolar disorder. Bipolar disorder also may be found in geographic, presumably genetic, isolates and in higher prevalence in higher socio-economic groups, which also differentiates it from schizophrenia. It is hypothesized that a subset of individuals with bipolar disorder constitutes a distinct disease entity, but that the majority share some common etiological antecedents with schizophrenia and may represent a disease continuum.