Depression is a mood disorder that ranges from mild to severe. All of us have experienced a form of depression at some time in our lives. It can accompany loss, stress, unresolved anger, fear, shame, disappointment and other emotional dynamics that are a part of our life situation. Depression is experienced by children, adults, and elderly regardless of economic status, gender, or culture. Depression affects physical health, relationships, awareness, weight, memory, decision making ability, energy level, interest, and sleep. In addition to emotional experience, depression can result from diet, nutrition, and other physical biochemical origins.

The Ancient Greek physician Hippocrates described a syndrome of melancholia as a distinct disease with particular mental and physical symptoms; he characterized all “fears and despondencies, if they last a long time” as being symptomatic of the ailment. It was a similar but far broader concept than today’s depression; prominence was given to a clustering of the symptoms of sadness, dejection, and despondency, and often fear, anger, delusions and obsessions were included.

Major depressive episodes often resolve over time whether or not they are treated. Outpatients on a waiting list show a 10–15% reduction in symptoms within a few months, with approximately 20% no longer meeting the full criteria for a depressive disorder. The median duration of an episode has been estimated to be 23 weeks, with the highest rate of recovery in the first three months.

The three most common treatments for depression are psychotherapy, medication, and electroconvulsive therapy. Psychotherapy is the treatment of choice for people under 18, while electroconvulsive therapy is only used as a last resort. Care is usually given on an outpatient basis, while treatment in an inpatient unit is considered if there is a significant risk to self or others.

The biopsychosocial model proposes that biological, psychological, and social factors all play a role to varying degrees in causing depression. The diathesis–stress model posits that depression results when a preexisting vulnerability, or diathesis, is activated by stressful life events. The preexisting vulnerability can be either genetic, implying an interaction between nature and nurture, or the result of past experience such as learned views of the world formed in childhood. These interactive models to understanding the causes of depression have gained empirical support. For example, a prospective, longitudinal study uncovered a moderating effect of the serotonin transporter (5-HTT) gene on stressful life events in predicting depression. Specifically, depression may follow such events, but is more likely to in people with one or, even more so, two short alleles of the 5-HTT gene. A Swedish study estimated the heritability of depression (the degree to which individual differences in occurrence are associated with genetic differences) to be approximately 40% for women and 30% for men.