The diagnosis of major depressive disorder is based on the patient's self-reported experiences, behavior reported by relatives or friends, and a mental status exam. There is no laboratory test for major depression, although physicians generally request tests for physical conditions that may cause similar symptoms. The most common time of onset is between the ages of 30 and 40 years, with a later peak between 50 and 60 years. Major depression is reported about twice as frequently in women as in men, and women attempt suicide more often, although men are at higher risk for completing suicide.

Most patients are treated in the community with antidepressant medication and some with psychotherapy or counselling. Hospitalization may be necessary in cases with associated self-neglect or a significant risk of harm to self or others. A minority are treated with electroconvulsive therapy (ECT), under a short-acting general anaesthetic. The course of the disorder varies widely, from one episode lasting months to a lifelong disorder with recurrent major depressive episodes. Depressed individuals have shorter life expectancies than those without depression, in part because of greater susceptibility to medical illnesses and suicide. Current and former patients may be stigmatized.

The understanding of the nature and causes of depression has evolved over the centuries, though this understanding is incomplete and many aspects of depression are still the subject of discussion and research. Psychological, psycho-social, hereditary, evolutionary and biological causes have been proposed. Psychological treatments are based on theories of personality, interpersonal communication, and learning. Most biological theories focus on the monoamine chemicals serotonin, norepinephrine, and dopamine, which are naturally present in the brain and assist communication between nerve cells. Monoamines have been implicated in depression, and most antidepressants work to increase the active levels of at least one.

Major depression is a serious illness that affects a person's family and personal relationships, work or school life, sleeping and eating habits, and general health.[1] Its impact on functioning and well-being has been equated to that of chronic medical conditions such as diabetes.[2]

A person suffering a major depressive episode usually exhibits a very low mood, which pervades all aspects of life, and an inability to experience pleasure in activities that formerly were enjoyed. Depressed people may be preoccupied with, or ruminate over, thoughts and feelings of worthlessness, inappropriate guilt or regret, helplessness, hopelessness, and self-hatred.[3] In severe cases, depressed people may have symptoms of psychosis. These symptoms include delusions or, less commonly, hallucinations, usually of an unpleasant nature.[4] Other symptoms of depression include poor concentration and memory (especially in those with melancholic or psychotic features),[5] withdrawal from social situations and activities, reduced sex drive, and thoughts of death or suicide.

Insomnia is common among the depressed. In the typical pattern, a person wakes very early and is unable to get back to sleep.[6] Hypersomnia, or oversleeping, is less common.[6] Appetite often decreases, with resulting weight loss, although increased appetite and weight gain occasionally occur.[3] The person may report multiple physical symptoms such as fatigue, headaches, or digestive problems; physical complaints are the most common presenting problem in developing countries, according to the World Health Organization's criteria for depression.[7] Family and friends may notice that the person's behavior is either agitated or lethargic.[6]

Depressed children often display an irritable rather than a depressed mood,[3] and show varying symptoms depending on age and situation.[8] Most exhibit a loss of interest in school and a decline in academic performance. They may be described as clingy, demanding, dependent, or insecure.[6] Diagnosis may be delayed or missed when symptoms are interpreted as normal moodiness.[3] Depression may also coincide with attention-deficit hyperactivity disorder (ADHD), complicating the diagnosis and treatment of both.[9]

Older depressed persons may have cognitive symptoms of recent onset, such as forgetfulness,[5] and a more noticeable slowing of movements.[10] Depression often coexists with physical disorders common among the elderly, such as stroke, other cardiovascular diseases, Parkinson's disease, and chronic obstructive pulmonary disease.[11]