Depression is a psychological disorder defined by persistent feelings of sadness, emptiness, and hopelessness. At current estimates, approximately one in five people will have an episode of depression in their lifetime; however, this varies by gender with a ratio of one in four women and one in eight men.

According to the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders IV (DSM IV), there are three main categories of depression: major depressive disorder (MDD), dysthymia, and bipolar disorder. Additionally, there are a number of seasonal and onset specifiers, producing subcategories of depression including seasonal affective disorder, postpartum depression, and premenstrual dysphoric disorder.

Depression rates are increasing worldwide with the World Health Organization (WHO) estimating that by 2020, depression will be the second largest contributor ...