In the fields of psychology and psychiatry, the terms depression or depressed refer to both expected and pathologically chronic or severe levels of sadness, perceived helplessness, disinterest, and other related emotions and behaviours. The Diagnostic and Statistical Manual of Mental Disorders (DSM) states that a depressed mood is often reported as being: "depressed, sad, hopeless, discouraged, or 'down in the dumps'." In traditional colloquy, "depressed" is often synonymous with "sad," but both clinical and non-clinical depression can also refer to a conglomeration of more than one feeling. Such a mixture can include (but is not limited to) anger, fear, anxiety, despair, guilt, apathy, and/or grief, in addition to what many people would describe as typical "sadness."
Determinants of mood
Depression can be the result of many factors, individually and acting in concert.
Reactions to events, often a loss in some form, are perhaps the most obvious causes. This loss may be obvious, such as the death of a loved one, or having moved from one house to another (mainly with children), or less obvious, such as disillusionment about one's career prospects. Monotonous environments can be depressing. Merely painting a workplace can stimulate productivity. A lack of control of one's environment can lead to feelings of helplessness. Domestic disputes and financial difficulties are common causes of a depressed mood. Other causes of depression are climatic conditions, such as a rainy weather and a lack of sunlight, loneliness, and feelings that one isn't cared about by others.
Internal psychological factors
Sometimes the depressed mood may relate more to internal processes or even be triggered by them. Pessimistic views of life or a lack of self-esteem can lead to depression. Illnesses and changes in cognition that occur in psychoses and dementias, to name but two, can lead to depression.
Biological models of causation
Adaptive benefits of depression
While a depressed mood is usually seen as deleterious, it may have adaptive benefits. Of interest is the fact that physical illness tends to lead to depressive behavior and some diseases, such as influenza, are often accompanied by a degree of depression that seems out of proportion to the physical illness. A depressed mood is adaptive in illness in that it leads to the person resting and in general elicits care. Seasonal affective disorder may point to an atavistic link with behavior in hibernation.
Biological influences of depression are varied, but can include hereditary, hormonal, and seasonal factors, stress, illness, neurotransmitter malfunction, and long-term exposure to dampness and mold and to aerosol exposure via the frequent use of air fresheners and other aerosols in the home,, all of which are more fully discussed in the major depressive disorder article.
Depression as mechanism of adaptation
While a depressed mood is usually referred to (and perceived) as negative, it can sometimes be subtly beneficial in helping a person adapt to circumstance. For example, physical illness, such as influenza, can lead to feelings of psychological malaise and depression that seem, at first, only to compound an already unpleasant situation. However, the experience of depression, or feeling "down," often results in physical inertia, which leads to the compulsion to rest. The fleeting helplessness and immobility of the physically ill may also serve to elicit care from others.
From an evolutionary standpoint, some argue that depression could be at least partially related to atavistic fears that were originally based on real dangers. Paul Keedwell, in his book, How Sadness Survived: The Evolutionary Basis of Depression, suggests that, because "social support and interdependence were important features of the [human] ancestral environment," "the [peer] group could have offered extra help to the depressed person until the condition resolved." Further, "...a depressed person may change the attitudes of other people around him, making them more sympathetic to his needs and therefore giving him a long term [social or reproductive] advantage."
Temporary depression, psychologist Thomas Moore, Ph.D., suggests, can, in some cases, not only "...provide a rest from the hyperactivity of the good times...," but can also be assigned value in the overall spectrum of human experience, and might enrich the ways in which members of a community relate to, and support, one another. In some cases, Moore says, "dark times [can] leave their mark and make you a person of insight and compassion." 
Psychological disorders with depression
Episodes of depressed mood are a core feature of the following psychological disorders, as specified by the DSM-IV:
- Major depressive disorder
- Bipolar disorder
- Schizoaffective disorder
- Seasonal affective disorder
- Adjustment disorder with depressed mood
Mental disorders with depression
A depressed mood is usually a core feature of some mental disorders such as:
- manic depression (Bipolar disorder)
- clinical depression
- endogenous depression
- reactive or neurotic depression
- atypical depression
- psychotic depression
- seasonal affective disorder
- adjustment disorder with depressed mood
- Obsessive-Compulsive Disorder (O.C.D.) Can often lead to depression if left untreated
Substances commonly used or promoted to treat depression (effective or not)
- Mozilla Open Directory: Depression - categorized links relating to the topic of depression.
- About the Brain – Includes definitions relating to depression
- beyondblue – The Australian National Depression Initiative
- Black Dog Institute – Depression and Bipolar Disorder Information Australia
- Depression at WebMD - Drug and treatment information for depression.
- Depression Research News at ScienceDaily
- Mental Health Disorders: Mood: Depression at Mozilla Open Directory (categorized links)
- National Alliance on Mental Illness – Depression support, advocacy, and education
- National Depressive and Manic Depressive Association - National Depressive and Manic Depressive Association
- Stanford Depression Research Clinic
- Substance Abuse and Mental Health Services (SAMHSA) – United States Department of Health and Human Services (HHS)
- Dampness and Mold in the Home and Depression: An Examination of Mold-Related Illness and Perceived Control of One’s Home as Possible Depression Pathways Edmond D. Shenassa, ScD, Constantine Daskalakis, ScD, Allison Liebhaber, BA, Matthias Braubach, MPH and MaryJean Brown, ScD, RN October 2007, Vol 97, No. 10 | RESEARCH AND PRACTICE | American Journal of Public Health 1893-1899 © 2007 American Public Health Association DOI: 10.2105/AJPH.2006.093773 PMID 17761567 http://www.ajph.org/cgi/content/abstract/97/10/1893?HITS=10&sortspec=relevance&hits=10&author1=Edmond+Shenassa&maxtoshow=&FIRSTINDEX=0&resourcetype=HWCIT&searchid=1&RESULTFORMAT=
- "Symptoms of mothers and infants related to total volatile organic compounds in household products" Arch Environ Health. 2003 Oct;58(10):633-41; PMID 15562635; "Air fresheners can make mothers and babies ill" University of Bristol press release issued 19 October 2004 http://www.news-medical.net/?id=5680
- How Sadness Survived: The Evolutionary Basis of Depression, Marcello Spinella | Radcliffe Publishing |2008| ISBN 1846190134
- Dark Nights of the Soul: A Guide to Finding Your Way Through Life's Ordeals, Thomas Moore, Ph.D.|Gotham Books|2004|ISBN 1592400671