A lot is being asked of the genetic analysis of major depression (MD): to find the biological underpinnings of one of the commonest psychiatric illnesses and one of the world’s leading causes of morbidity. While lifetime prevalence estimates vary, from 3% in Japan to 16.9% in the U.S., in all countries the disorder is common, with a frequency typically varying from 8% to 12% (Demyttenaere et al., 2004, Kessler et al., 2003). In the U.S., MD has the greatest impact of all biomedical diseases on disability; in Europe, it is the third leading cause of disability (Alonso et al., 2004b, Nierenberg et al., 2001, Penninx et al., 2001, Ustün et al., 2004).
