A 32-yrs-old woman is diagnosed with major depression. What is the chance that her identical twin sister will develop the same disease?

Twin studies are useful tools to separate biological and environmental etiologic factors in illnesses. Most twin studies sta with the selection of monozygotic and same-sex dizygotic twin pairs who are raised together, in which one of the twins has the disorder. The other twin is then followed to determine if he or she also develops the disorder. The propoion of the twin pairs in which both twins are affected is called concordance. The difference in the concordance rate between the monozygotic and the dizygotic pairs reflects the impoance of hereditary factors in the development of the disorder. The concordance rate for mood disorders in monozygotic twins averages 70% when various studies are combined. The concordance rate in Dizygotic twins is approximately 20%. This is significantly higher than that found in the general population, but still 2 to 4 times lower than the concordance in monozygotic twins. These studies prove that, although environmental factors do play a role in mood disorders (the concordance is not 100%), heredity has a major role. Reference: Kaplon and sadock, 11 th edition, synopsis of psychiatry, 11 th edition, pg no.835