|Classification and external resources|
Depression is a state of low mood and aversion to activity that can affect a person's thoughts, behavior, feelings and sense of well-being. People with depressed mood can feel sad, anxious, empty, hopeless, helpless, worthless, guilty, irritable, ashamed or restless. They may lose interest in activities that were once pleasurable, experience loss of appetite or overeating, have problems concentrating, remembering details or making decisions, and may contemplate, attempt or commit suicide. Insomnia, excessive sleeping, fatigue, aches, pains, digestive problems or reduced energy may also be present. Depressed mood is a feature of some psychiatric syndromes such as major depressive disorder, but it may also be a normal reaction to life events such as grief, a symptom of some bodily ailments or a side effect of some drugs and medical treatments.
Adversity in childhood, such as grief, neglect, mental abuse, and unequal parental treatment of siblings can contribute to depression in adulthood. Childhood physical or sexual abuse in particular significantly increases the likelihood of experiencing depression over the life course.
Life events and changes that may precipitate depressed mood include childbirth, menopause, financial difficulties, job problems, a medical diagnosis (cancer, HIV, etc.), bullying, loss of a loved one, natural disasters, social isolation, relationship troubles, jealousy, separation, and catastrophic injury. Adolescents may be especially prone to experiencing depressed mood following social rejection.
Certain medications are known to cause depressed mood in a significant number of patients. These include interferon therapy for hepatitis C, medications for high blood pressure and sleep inducing pills. 
Depressed mood can be the result of a number of infectious diseases, nutritional deficiencies, neurological conditions  and physiological problems, including hypoandrogenism (in men), Addison's disease, Lyme disease, multiple sclerosis, chronic pain, stroke, diabetes, and cancer.
A number of psychiatric syndromes feature depressed mood as a main symptom. The mood disorders are a group of disorders considered to be primary disturbances of mood. These include major depressive disorder (MDD; commonly called major depression or clinical depression) where a person has at least two weeks of depressed mood or a loss of interest or pleasure in nearly all activities; and dysthymia, a state of chronic depressed mood, the symptoms of which do not meet the severity of a major depressive episode. Another mood disorder, bipolar disorder, features one or more episodes of abnormally elevated mood, cognition and energy levels, but may also involve one or more episodes of depression. When the course of depressive episodes follows a seasonal pattern, the disorder (major depressive disorder, bipolar disorder, etc.) may be described as a seasonal affective disorder.
Outside the mood disorders: borderline personality disorder often features an extremely intense depressive mood; adjustment disorder with depressed mood is a mood disturbance appearing as a psychological response to an identifiable event or stressor, in which the resulting emotional or behavioral symptoms are significant but do not meet the criteria for a major depressive episode;:355 and posttraumatic stress disorder, an anxiety disorder that sometimes follows trauma, is commonly accompanied by depressed mood. Depression is sometimes associated with substance use disorder. Both legal and illegal drugs can cause substance use disorder.
Depressed mood may not require any professional treatment, and may be a normal reaction to certain life events, a symptom of some medical conditions, or a side effect of some drugs or medical treatments. 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. Different sub-divisions of depression have different treatment approaches. In the United States, it has been estimated that two thirds of people with depression do not actively try to receive 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.
There are a number of symptoms that are used to help diagnose depression. Looking at all instances of depression that can be confirmed (i.e. psychological help is obtained or a suicide occurs from depression-related circumstances), a one-author report states that women have a higher rate of major depression than men, but looking at individual symptoms, the gender divide intensifies in some areas, disappears in some, and reverses in others. While women have a greater proportion of somatic symptoms, such as appetite, sleep disturbances and fatigue accompanied by pain and anxiety, than men, the gender difference is much smaller in other aspects of depression. Female respondents report twice the prevalence of somatic symptoms as male (2.8% vs. 1.4%) versus depression not associated with somatic symptoms (2.3% vs. 1.7%). Depression with somatic symptoms is highly likely to also have an anxiety disorder (31.4% vs. 22.9%), to have pain (60% vs. 48%), and to have chronic depression (49.2% vs. 36.8%). Men with depression with somatic symptoms were more likely than those without to have pain (48.9% vs. 28.6%) but were not more likely to have an anxiety disorder (39.3% vs. 31.9%) or chronic dysphoria (37.8% vs. 33.3%). Instances of suicide in men is much greater than in women. In a report by Lund University in Sweden and Stanford University, it was shown that men commit suicide at a rate almost three times that of women in Sweden, and the Centers for Disease Control and Prevention and National Center for Injury Prevention and Control report that the rate in the US is almost four times as many males as females. However, women have higher rates of suicide ideation and attempts. The difference is attributed to men choosing more effective methods resulting in the higher rate of success.
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A problem in analyzing the results of mental health research and mental health statistics is the prevalence of reporting bias. Currently, the only way to collect data is to survey people and look at hospitalization rates. There are potential problems with people mis-reporting their feelings, the frequency of depressive and anxious thoughts, and other information that subjects might want to suppress or might not realize are significant. This is exacerbated by cultural norms that encourage males to not express their feelings and to “tough it out” while women are encouraged to express emotions.
There is also the tendency to view mental illness as "all in your mind," with the preconception that the problem can be solved by just trying hard enough. Subjects may underreport the prevalence of depressive or anxious episodes, men more so than women for the above reason. Because of perceived stigma, subjects may resist the idea that they might have a mental illness and attempt to suppress any hint to the contrary on a survey.
While women are diagnosed with depression and anxiety statistically more often than men, there is societal pressure on all genders to not report mental illnesses because they are afraid of being viewed as "crazy", or are afraid of being judged as weak and powerless to control themselves. This leads to potentially significant errors in statistical analyses of the prevalence of internalized mental disorders, and potential errors in the judgement of differences experienced by gender.
Finally, because of societal pressures, people suffering from mental disorders or emotional trauma because of, for example, sexual assault or abuse may feel as though no one will support them even if they do report it—a form of learned helplessness. This may happen to all genders, but it is hypothesized that different genders and gender identities are affected disproportionately depending on the specific situation.
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