[6] People with depression may experience sadness, feelings of dejection or hopelessness, difficulty in thinking and concentration, or a significant change in appetite or time spent sleeping; suicidal thoughts can also be experienced.

[8][9] Childhood physical or sexual abuse in particular significantly correlates with the likelihood of experiencing depression over the survivor's lifetime.

[11] Poor housing quality, non-functionality, lack of green spaces, and exposure to noise and air pollution are linked to depressive moods, emphasizing the need for consideration in planning to prevent such outcomes.

[13] Likewise, those from smaller towns and rural areas tend to have higher rates of depression, anxiety, and psychological unwellness.

[15][16][17] Life events and changes that may cause depressed mood includes, but are not limited to, childbirth, menopause, financial difficulties, unemployment, stress (such as from work, education, military service, family, living conditions, marriage, etc.

), bullying, loss of a loved one, natural disasters, social isolation, rape, relationship troubles, jealousy, separation, or catastrophic injury.

[37][38][39] A 2014 study by Hooley et al. concluded that chronic pain increased the chance of death by suicide by two to three times.

[48] Authors and researchers have begun to conceptualize ways in which the historical legacies of racism and colonialism may create depressive conditions.

Cox, Abramson, Devine, and Hollon are concerned with the ways in which social stereotypes are often internalized, creating negative self-stereotypes that then produce depressive symptoms.

Unlike the theory of "deprejudice", a psychogeographical theory of depression attempts to broaden study of the subject beyond an individual experience to one produced on a societal scale, seeing particular manifestations of depression as rooted in dispossession; historical legacies of genocide, slavery, and colonialism are productive of segregation, both material and psychic material deprivation,[53] and concomitant circumstances of violence, systemic exclusion, and lack of access to legal protections.

The demands of navigating these circumstances compromise the resources available to a population to seek comfort, health, stability, and sense of security.

The historical memory of this trauma conditions the psychological health of future generations, making psychogeographical depression an intergenerational experience as well.

Beck's cognitive triad theorizes that an individual with depression has "automatic, spontaneous, and seemingly uncontrollable negative thoughts"[56] about the self, the world or environment, and the future.

A prolonged depressed mood, especially in combination with other symptoms, may lead to a diagnosis of a psychiatric or medical condition which may benefit from treatment.

The UK National Institute for Health and Care Excellence (NICE) 2009 guidelines indicate that antidepressants should not be routinely used for the initial treatment of mild depression, because the risk-benefit ratio is poor.

[61][62] Reminiscence of old and fond memories is another alternative form of treatment, especially for the elderly who have lived longer and have more experiences in life.

[63] It is a method that causes a person to recollect memories of their own life, leading to a process of self-recognition and identifying familiar stimuli.

By maintaining one's personal past and identity, it is a technique that stimulates people to view their lives in a more objective and balanced way, causing them to pay attention to positive information in their life stories, which would successfully reduce depressive mood levels.

[64] There is limited evidence that continuing antidepressant medication for one year reduces the risk of depression recurrence with no additional harm.

Its consequences further lead to significant burden in public health, including a higher risk of dementia, premature mortality arising from physical disorders, and maternal depression impacts on child growth and development.

Derived from the Ancient Greek melas, "black", and kholé, "bile",[82] melancholia was described as a distinct disease with particular mental and physical symptoms by Hippocrates in his Aphorisms, where he characterized all "fears and despondencies, if they last a long time" as being symptomatic of the ailment.

[83] During the 18th century, the humoral theory of melancholia was increasingly being challenged by mechanical and electrical explanations; references to dark and gloomy states gave way to ideas of slowed circulation and depleted energy.

[84] German physician Johann Christian Heinroth, however, argued melancholia was a disturbance of the soul due to moral conflict within the patient.

[85] In July 2022, British psychiatrist Joanna Moncrieff, also psychiatrist Mark Horowtiz and others proposed in a study on academic journal Molecular Psychiatry that depression is not caused by a serotonin imbalance in the human body, unlike what most of the psychiatry community points to, and that therefore anti-depressants do not work against the illness.