It is important to mention that that the DSM- IV attempts to differentiate depression from normal sadness along the lines of intensity, absence of precipitants, quality, associated features, and history. This means that the depressive mood must pervade all aspects of the individual's life, and usually leads to social and occupational dysfunction. It is not an expected reaction to external stimuli, but a reaction that is extremely out of proportion, or an onset of a mood that occurs with no precursor at all. The mood change is inherently different from that of normal sadness; it may be accompanied by other symptoms or be preceded by a history of similar episodes. Major depression is the "common cold" of mood disorders because it is the one from which most individuals are likely to suffer. It does not seem to discriminate according to socioeconomic standing. Men have a 5 to 10 percent lifetime prevalence of developing the disorder. Their point prevalence, or the number of men likely to be diagnosed with depression at any time, is approximately 2 to 3 percent. The lifetime prevalence of women who will meet the criteria for major depression is 10 to 20 percent; the point prevalence ranges from 5 to 9 percent. Therefore, women (whether this is a reflection of biased data collection or the bias of women being more likely to seek treatment and be labeled as depressive) show a higher prevalence rate than men for unipolar mood disorder (also called major, or clinical, depression). Clinical depression, contrary to what one might expect, is also less likely to appear in the elderly than in younger adults. Unipolar mood disorders usually have an age of onset between twenty and forty years of age, with depressive episodes lasting of a minimum duration of two weeks, although they can be much longer.

With or without medication, subjects tend to improve within six months within the beginning of a major depressive episode. Of those who recover, 40 percent relapse within a year. If the disorder goes untreated, these individuals will remit within six to twelve months; if treated, the disorder remits in about twelve weeks. About 50 percent of those in remission, regardless of treatment, will relapse. However, the risk of relapse does tend to decrease as the period of remission increases. In about 10 percent of the cases, individuals will depress into manic or hypomanic stages and can then be classified as having bipolar mood disorder. Unfortunately, there are no cures for these disorders, but only methods of shortening the length of the episodes. Of those individuals who suffer from major depression, about 18 percent end their life in suicide. Cross- cultural studies indicate that clinical depression is a universal phenomenon, although symptoms may vary from one culture to another, and is highly comorbid with many other psychological disorders such as anxiety and alcoholism.

The diagnostic criteria of the DSM-IV for major depression includes a combination of emotional, cognitive, somatic and behavioral symptoms that are recurrent and pervasive. One of the most prominent emotional symptoms consists of a dysphoric, or unpleasant, mood. This entails feelings of gloom, dejection, and despondence. Feelings of anxiety are also common amongst people suffering from depression (similarly, depression is a common feature of many anxiety disorders. The cognitive symptoms refer to changes in the way the individual feels about him- or herself and his or her surroundings. Depressed individuals are usually characterized by trouble concentrating and by easily being distracted. Guilt and worthlessness are also common cognitive features of depressed individuals. Not only do they tend to blame themselves more for failure, even in the face of contradicting evidence, but they also tend to focus attention on the negative features of themselves, their environments and their future (labeled by Aaron Beck as the "depressive triad"). Depression also leads to the exhibition of avolition, or loss of willpower, and general tendencies of indecisiveness and ambivalence. Depressed individuals are also usually occupied with suicidal ideas.

The somatic symptoms of depression concern changes in psychological functions. These include changes in sleep pattern--inability to sleep and waking early, or sleeping too much, and changes in appetite--usually unintentional weight loss, or, less commonly, weight gain. Bodily symptoms also include fatigue or loss of energy, headaches, and muscular aches and pains. Finally, depression may be characterized by loss of interest in activities and sources of pleasure, also known as anhedonia, such as a decrease in sexual activity and enjoyment of it. The behavioral symptoms that contribute to a diagnosis of depression fall under the category of psychomotor retardation; depressed individuals are more likely to walk and talk at slower rates than usual. In total, five of the nine symptoms mentioned must be experienced to meet the criteria for a diagnosis of major depression.

Dysthymia is a mild form of depression that is characterized by a chronic course and differs from major depression in terms of both severity and duration. To meet the diagnostic criteria of dysthymia, the individual, must, over a period of at least two years, exhibit a depressed mood for most of the day on more days than not, and this mood cannot be absent for more than two months at any time in the two year period. If the person at any time in the two-year period experiences a depressive episode, the diagnosis would be major depression or bipolar mood disorder (if the individual also experiences a manic episode within this period.