Understanding Depression -- the Basics

Medically Reviewed by Smitha Bhandari, MD on September 14, 2023
6 min read

Almost all of us feel low sometimes, usually because of a disturbing or difficult event in our lives. But ongoing sadness or despair can be caused by depression, a serious condition that warrants treatment. Talking to your primary care doctor can be a good place to start. Your doctor can screen you for depression and help treat your symptoms.

Depression affects more than 300 million people worldwide -- 20% of all women, 10% of all men, and 5% or more of all adolescents. It is the leading cause of disability worldwide, according to the World Health Organization, and is the second most common psychiatric problem in the U.S. (after anxiety disorders), afflicting about 17.6 million people each year at a cost in the range of about $50 billion a year. 

Depression can strike at any age, including in childhood. Between 2013 and 2016, 8.1% of American adults ages 20 and older had depression in a given 2-week period. In 2017, 13.3% of teens ages 12 to 17 reported at least one major depressive episode in the previous year. Among children ages 3 to 17, about 3.2% have diagnosed depression.

However, most people first experience depression when they are in their early thirties, and depression is particularly rife among older adults. Depression is not simply a normal reaction to the challenges of growing older, such as the death of a spouse or friends and the physical limitations of age, but is a medical condition without a known cause.

About 15% of women experience postpartum depression after giving birth. In these cases, symptoms may last from weeks to years. With professional help, almost all women who experience postpartum depression are able to overcome their symptoms.

  • Depressive reaction. A less-severe and often temporary depression that arises from a specific life situation. In modern diagnostic language, a depressive reaction to a specific life stress is technically called a "stress response syndrome" (formerly known as an "adjustment disorder with depressed mood"). The symptoms can be severe, but unless they involve additional symptoms such as changes in sleep and appetite or thoughts of suicide, they usually do not need medication treatment and will abate over time -- anywhere from two weeks to six months. Psychotherapy is sometimes recommended if symptoms start to interfere with normal everyday functioning.
  • Major depression. A serious condition that can lead to an inability to function or suicide. Sufferers experience not only a depressed mood, but also have difficulty performing simple daily tasks, lose interest in their usual activities, extreme fatigue, sleep problems, or feelings of guilt and helplessness. They can sometimes also lose touch with reality, having delusions (such as believing they have committed a sin, or are dying) or hallucinations (such as hearing an imaginary voice telling them they are no good), in severe cases. It can be a cyclical illness, so while most patients recover from their first depressive episode, the recurrence rate is high -- perhaps as high as 60% within two years and 75% within 10 years. After 15 years, 90% of individuals will have suffered a recurrence or relapse of depression.

Major depression, which affects more than 16% of U.S. adults over a lifetime, often appears spontaneously and is seemingly unprovoked, or it can begin as a depressive reaction following a loss, trauma, or other significant stressful event. In people who are biologically predisposed to developing a depressive illness, the initial depressive reaction can intensify and evolve into a clinically full-blown depressive episode. The depressive episode may also disappear spontaneously, usually within six to 12 months, although medication as well as other forms of treatment are often needed to achieve full control of symptoms. Because of its disabling effects and the possibility of suicide, major depression often requires medical treatment.

  • Dysthymia. A low-grade, long-term depression that lasts for more than one year for children and adolescents and at least two years for adults. Dysthymia involves fewer symptoms than occur in a major depressive episode, but it is persistent and longstanding and often can be as disabling as major depression. Over the course of a lifetime, over 11% of teens (13-18) suffer from dysthymia, according to the National Institute of Mental Health. In modern diagnostic terminology, dysthymia together with chronic major depression (that is, a major depressive episode lasting two years or longer) are both included under the category of "persistent depressive disorder."


No one knows exactly what causes depression, although it appears to be an illness that may result from the interplay of many biological and environmental factors. Depressive reactions, which can involve sad mood but not the physical signs and symptoms of a major depressive episode, occur as a result of a particular event. Depressed moods can also be a side effect of medication, hormonal changes (such as before menstrual periods or after childbirth), or a physical illness, such as the flu or a viral infection. Clinical depression involves a syndrome of many physical and emotional or behavioral symptoms that can occur for no apparent reason in people who are biologically vulnerable to the disorder.

Although the exact causes of major depression and dysthymia are unknown, researchers currently believe that both of these forms of depression are caused by a malfunction of brain circuits that regulate mood, thinking and behavior. Brain chemicals called neurotransmitters (such as serotonin, norepinephrine and dopamine) are important for healthy nerve cell connections; medicines that can regulate the levels and functioning of these chemicals can help to fine-tune the efficiency of how these brain circuits function.

Brain "malfunctions" related to depression can have a genetic component, although genetics alone does not fully explain the risk or emergence of clinical depression. In one study, 27% of depressed children had close relatives who suffered from mood disorders.

What Are the Risk Factors for Depression?

Many factors or a combination of factors can increase the chance of depression, or make it more difficult to treat if it occurs, including:

  • Abuse. Past physical, sexual, or emotional abuse has been associated with depression later in life among people who may be biologically predisposed to depression.
  • Certain medications. For example, some drugs used to treat high blood pressure or liver disease can increase your risk of depression.
  • Conflict. Depression may sometimes be triggered by personal conflicts or disputes with family members or friends.
  • Death or a loss. Sadness or grief from the death or loss of a loved one, though natural, can also increase the risk of depression in people who are biologically predisposed to developing it.
  • Genetics. A family history of depression may increase the risk. It's thought that depression is sometimes passed genetically from one generation to the next, similar to other complex diseases that can run in families, such as diabetes, heart disease, and cancer. The exact way this happens, though, is not known. Genetics alone, however, does not fully explain the occurrence of depression.
  • Major events. Even positive events such as starting a new job, graduating, or getting married, can lead to depression. So can moving, losing a job or income, getting divorced, or retiring.
  • Other personal problems. Problems such as social isolation due to other mental illnesses or being cast out of a family or social group can lead to depression.
  • Serious illnesses. Sometimes depression coexists with a major illness or may be triggered by a reaction to the illness.
  • Substance abuse. Nearly 30% of people with substance abuse problems also have major or clinical depression.