Multinational studies indicate that the lifetime risk of bipolar disorder is approximately 1-2%. Depending on how the phenotype is defined, the concordance rate for bipolar illness ranges from 65% to 85% in monozygotic twins and is 20% in dizygotic twins. Bipolar illness occurs in relatives of patients with bipolar disorder much more frequently than in relatives of patients with major depression; the rates for depression alone are approximately the same. Adoption studies show that rates of illness clearly depend on the risk associated with the biological rather than the adoptive parents. In general, bipolar probands have more relatives with bipolar disorder and mood disorder than unipolar probands.
