Depression is a state of low mood and aversion to activity. While often described as a dysfunction, there are also strong arguments for seeing depression as an adaptive defense mechanism.
The Diagnostic and Statistical Manual of Mental Disorders defines a depressed person as experiencing feelings of sadness, helplessness and hopelessness. In traditional colloquy, feeling "depressed" is often synonymous with feeling "sad", but both clinical depression and non-clinical depression can also refer to a conglomeration of more than one feeling.
Biological influences of depression are varied, but may relate to malnutrition, heredity, hormones, seasons, stress, illness, drug or alcohol use, neurotransmitter malfunction, long-term exposure to dampness and mold, back injury, and to aerosol exposure. There are also correlations between long term sleep difficulties and depression. Up to 90% of patients with depression are found to have sleep difficulties.
A number of authors have suggested that depression is an evolutionary adaptation. A low or depressed mood can increase an individual's ability to cope with situations in which the effort to pursue a major goal could result in danger, loss, or wasted effort. In such situations, low motivation may give an advantage by inhibiting certain actions. This theory helps to explain why depression is so prevalent, and why it so often strikes people during their peak reproductive years. These characteristics would be difficult to understand if depression were a dysfunction, as many psychiatrists assume.
Depression is a predictable response to certain types of life occurrences, such as loss of status, divorce, or death of a child or spouse. These are events that signal a loss of reproductive ability or potential, or that did so in humans' ancestral environment. Depression can be seen as an adaptive response, in the sense that it causes an individual to turn away from the earlier (and reproductively unsuccessful) modes of behavior.
A depressed mood is common during illnesses, such as influenza. It has been argued that this is an evolved mechanism that assists the individual in recovering by limiting his/her physical activity. The occurrence of low-level depression during the winter months, or seasonal affective disorder, may have been adaptive in the past, by limiting physical activity at times when food was scarce. It is argued that humans have retained the instinct to experience low mood during the winter months, even if the availability of food is no longer determined by the weather.
An alternative theory posits that depression is a plea for help. However this view is not widely credited by evolutionary biologists: depression is observed in other species that are not social, and depression in humans is often actively hidden from others; even when it is apparent, it often fails to elicit a positive response.
Milder depression has been associated with what has been called depressive realism, or the "sadder-but-wiser" effect, a view of the world that is relatively undistorted by positive biases.DSM-IV-TR text from mindsite.comNational Alliance on Mental IllnessDepression and Bipolar Support AllianceDepression and bipolar disorder information from Australia