The experience of depression likely preceded recorded history. Certainly it has been recognized for thousands of years. Aretaeus of Cappadocia (c. 81–138 A.D.) is credited with the first clinical description of depression. Hippocrates, the Greek physician of antiquity, was well aware of depression; he called it “melancholia.”

Although it presents with an array of physical symptoms, depression is considered a disorder of mood. It is also called an “affective disorder” to signify that one of its key aspects is a disturbance of emotions or feelings. Almost 20% of adults will have a mood disorder requiring treatment during their lifetime, and about 8% of adults will have a major depressive disorder. Depression is the leading cause of disability and premature death among people aged 18 to 44 years, and it is expected to be the second leading cause of disability for people of all ages by 2020.

The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) is the 2013 update to the American Psychiatric Association's (APA) classification and diagnostic tool. The DSM-5 supersedes the DSM-IV-TR, which was published in 2000.

The American Psychiatric Association’s DSM-5 (2013) Diagnostic and Statistical Manual of Mental Disorders, states that the diagnosis of depression depends on the presence of two cardinal symptoms: (1) persistent and pervasive low mood, and (2) loss of interest or pleasure in usual activities. Depressive symptoms are judged to be of clinical significance when they interfere with normal activities and persist for at least two weeks, in which case a diagnosis of a depressive illness or disorder may be made (DSM-5, 2013).

The DSM-5 (2013) defines two classes of mood disorders in adults: Bipolar and related disorders and depressive disorders. Bipolar, or manic-depressive, disorder is characterized by both a depressed and a manic component of mood. Some scientists think what we usually consider depression may be better characterized as a syndrome than a discrete disorder. A syndrome may have many causes that result in what appears to be the same constellation of symptoms (Winokur, 1997).

A number of changes occurred between DSM-4 and DSM-5 in understanding depression:

• The bereavement exclusion in DSM-IV was removed from depressive disorders in DSM-5.
• New disruptive mood dysregulation disorder (DMDD) was added for children up to age 18 years.
• Premenstrual dysphoric disorder was moved from an appendix for further study, and became a disorder.
• Specifiers were added for mixed symptoms and for anxiety, along with guidance to clinicians for suicidality.
• The term dysthymia  would also be called persistent depressive disorder.

The Disability Adjusted Life Years (DALYs) measure lost years of healthy life regardless of whether the years are lost to premature death or disability. Using the DALYs measure, major depression has been ranked second only to ischemic heart disease in magnitude of disease burden in developed countries (Murray and Lopez, 1996). This may be an understatement, however, because people with depression are at greater risk of developing heart disease (Nemeroff et al., 1998).

The cost of depression to society is large. Kessler and colleagues (1999) reported depressed workers had between 1.5 and 3.2 times more short-term work disability days in a 30-day period than nondepressed workers, with a salary-equivalent productivity loss averaging between $182 and $395. The Massachusetts Institute of Technology Sloan School of Management ranks depression among some of the most costly major health concerns—including coronary heart disease, cancer, and AIDS—and says that depression is more widespread than all three (Turner, 1995).

Many people assume that depression is primarily a problem that affects women. Yet men commit suicide at rates 4 to 18 times the rate for women. In order to prevent suicide and offer better treatment options for both men and women, we need a better understanding of depression as it relates to gender.