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Depression: Overview

Key points

  • Depression describes a range of conditions with symptoms of persistent low mood and reduced interest in day-to-day experiences.
  • For 10–15% of men and 15–25% of women, a depressed mood can persist and become a significant mental health problem, called clinical or major depression.
  • Genetic, psychological, biological and environmental factors may play a role in the onset of depression.

Types of depression

In major depressive disorder, symptoms must occur for at least two weeks. They must represent a change from previous behaviour or mood. Depressed mood or loss of interest must be present.

There are three subtypes of depression:

  • Seasonal affective disorder tends to be affected by the weather and time of year.
  • Postpartum depression occurs after the birth of a child.
  • Depression with psychosis involves a break with reality, in which the person experiences hallucinations (hearing voices or seeing people or objects that are not there) or delusions (beliefs that have no basis in reality).

The Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) includes a fourth subtype: dysthymia. Symptoms include a chronically low mood with some moderate symptoms of depression such as poor appetite or overeating, inability to sleep or sleeping too much, low energy or fatigue, low self-esteem, poor concentration, difficulty making decisions and feelings of hopelessness.

DSM-5 combines major depression and dysthymia into the diagnosis of persistent depressive disorder.


The chief symptom of major depression is a sad, despairing mood that persists beyond two weeks and impairs the person's performance at work or school, or in social relationships.

A diagnosis of major depression requires at least one of the core symptoms:

  • changes in appetite and weight
  • sleep problems
  • loss of interest in work, hobbies, people
  • feelings of uselessness, hopelessness, excessive guilt
  • preoccupation with failure or inadequacies, and a loss of self-esteem
  • agitation or loss of energy
  • slowed thinking, forgetfulness, trouble concentrating and making decisions
  • decreased sexual drive
  • a tendency to cry easily, or having the urge to cry
  • suicidal or occasionally homicidal thoughts
  • at times, a loss of touch with reality, perhaps hearing voices (hallucinations)
  • having strange ideas (delusions).

Co-occurring conditions

Depression is often secondary to, or comorbid with, many other mental health disorders. Bipolar disorder or manic-depressive illness is characterized by episodes of depression. Anxiety and personality disorders often also have features of depression.


Genetic and family history

People with a family history of depression have slightly higher chances of becoming depressed at some stage in their lives.

  • Studies of twins raised separately have shown that if one twin develops the disorder, the other has a 40–50% chance of also being affected. This finding suggests that some people have a genetic predisposition to developing depression.
  • The onset of depression may be influenced by what we learn as children. Some people may have been exposed to the depressive symptoms of their parents and have learned this as a way of reacting to certain problems. As adults, they may go on to use these strategies to deal with their own life stressors.

Psychological vulnerability

Personality style and the way people learn to deal with problems may contribute to the onset of depression.

Life events

Early childhood trauma and losses can lead to depression.

Experiencing several severe and prolonged difficult life events increases a person's risk of developing a depressive disorder.

Living with chronic family problems can seriously affect a person's mood and lead to depressive symptoms.

People living in emotionally abusive or violent relationships can feel trapped, financially and emotionally, and feel hopeless about their future. This is particularly true of mothers with young children.

Biological factors

Depression may appear after physiological events such as childbirth or having a viral or other infection. This observation has led to the theory that hormonal or chemical imbalances in the brain may cause depression.

Studies have shown that people with depression have different levels of certain biochemicals (e.g., serotonin) than people without depression. The fact that depression can be helped by antidepressant medication and electroconvulsive therapy tends to support this theory.


The outlook varies with the severity of the depression:

The average length of a depression episode is six to eight months. With mild depression, spontaneous recovery is likely.

About 80% of people treated for major depression will have at least one more episode in their lifetime. The average is four episodes.

Risk factors for recurrence of depression include:

  • a history of three or more episodes of major depression
  • a history of frequent episodes
  • an episode in the previous 12 months
  • residual symptoms
  • severe episodes, e.g., suicidality (likelihood of completing suicide), psychotic features
  • long previous episodes.

Evidence summary

American Psychiatric Association. (2000). Diagnostic and Statistical Manual of Mental Disorders (4th ed., tex rev.). Washington, DC: Author.

American Psychiatric Association. (2013). Diagnostic and Statistical Manual of Mental Disorders (5th ed.). Washington, DC: Author.

Kennedy, S.H., Lam, R.W., Parikh, S.V., Patten, S.B. & Ravindran, A.V. (2009). Canadian Network for Mood and Anxiety Treatments (CANMAT) Clinical Guidelines for the Management of Major Depressive Disorder in AdultsJournal of Affective Disorders, 117(Suppl. 1). S44–53. doi: 10.1016/j.jad.2009.06.039

National Institute for Health and Care Excellence. (2005). Depression in Children and Young People: Identification and Management in Primary, Community and Secondary Care (Clinical Guideline 28). London, UK: Author.

National Institute for Health and Care Excellence. (2009). Depression in Adults: The Treatment and Management of Depression in Adults (Clinical Guideline 90). London, UK: Author.

National Institute for Health and Care Excellence. (2009). Depression in Adults with a Chronic Physical Health Problem: Treatment and Management (Clinical Guideline 91). London, UK: Author.