Family, twin, and adoption studies of bipolar disease.

Bipolar disease features states of severe depression that usually fluctuate with at least one episode of intense elation or mania. It is a disorder that has been thought for some time to have a heritable component. The lifetime prevalence of bipolar disease in the general population is approximately 1%. In contrast, family studies have shown the approximate lifetime risk of a first-degree relative of a bipolar proband to be 5% to 10%. Moreover, studies of monozygotic twins show that their risk of contracting the disease is as much as 75 times greater than that for the general population. In addition, adoption studies have demonstrated that biological relatives of bipolar patients are substantially more likely to have the disorder than are adoptive relatives.