We recruited 419 MDD patients of age over 18 years from the Clinical Research Center for Depression study in South Korea. At the start of the study, the onset age of the first major depressive episode was self-reported by the subjects. The subjects were divided into four age-at-onset subgroups: childhood and adolescent onset (ages<18), early adult onset (ages 18-44), middle adult onset (ages 45-59), and late onset (ages 60+)