Depression Research Articles & Resources

Karina Thadani
Last updated:
Erin L. George, MFT
Erin L. George, MFT
Medical editor

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What Is Depression?

Depression is a serious mood disorder that negatively impacts thoughts, emotions, and behavior. Characterized by persistent sadness that lasts for at least 2 weeks, depression may be categorized as mild, moderate, or severe. (1)The most common type of depression is major depression. Also called clinical depression or unipolar depression, it's a moderate to severe level of sadness that is experienced most days of the week. According to the National Institute of Mental Health, over 21 million Americans have had major depression at some point. (2) Other types of depression include:

  • Seasonal Affective Disorder (SAD): Temporary period of unipolar depression experienced during winter
  • Persistent Depressive Disorder (Dysthymia): Depression that lasts for 2 years or longer
  • Postpartum Depression: Major depression that's experienced soon after childbirth
  • Manic Depression (Bipolar Disorder): Characterized by extreme mood swings
  • Premenstrual Dysphoric Disorder (PDD): Depression that occurs at the start of a woman's menstrual cycle

Although people may experience depression at different times and for different reasons, all types can seriously alter mood and behavior. While some types (such as SAD) tend to resolve on their own, most forms of depression will persist unless the person affected seeks treatment. (3)


What Causes Depression?

Some forms of depression have clear causes. For example, SAD is linked to winter, postpartum depression is associated with childbirth, and PDD is connected to menstrual cycles. For general major depression, however, the cause is less clear.
In the past, researchers believed clinical depression was connected to low levels of certain neurotransmitters, or chemicals that send messages between neurons in the brain. These neurotransmitters included serotonin (which regulates mood and sleep), dopamine (which promotes pleasure), and norepinephrine (which improves attention span). However, newer studies suggest that while depression patients may have chemical imbalances, they do not cause the condition. (4)

So, what causes depression? There are quite a few contributors.The disorder is believed to run in families, which suggests that some people may be genetically predisposed to depression. (5) An individual's environment can also play a role in whether they develop depression. Risk factors include the following:

A traumatic event can significantly increase a person's chances of experiencing depression. According to a study by the National Library of Medicine, nearly 50% of people with post-traumatic stress disorder (PTSD) also had depression. (6) Substance use, isolation, and disease diagnoses are also linked to high depression levels.

What Are the Symptoms of Depression? Signs to Know.

In cases of mild depression, symptoms may be confused for general sadness. As the disorder progresses, however, the symptoms of depression become more noticeable and start to interfere with day-to-day life. Common signs of depression include the following:

  • Persistent sadness or hopelessness
  • Sudden mood swings
  • Increased irritability
  • Loss of interest in previously enjoyable activities
  • Difficulty sleeping
  • Problems with concentration and memory
  • Fatigue and restlessness
  • Lower appetite and sudden weight changes
  • Chronic headaches and pains
  • Suicidal thoughts

Depression symptoms may impact a person's ability to attend school/work, engage in social relationships, and fulfill their daily obligations. Many people with the disorder choose to withdraw from others, which consequently worsens the symptoms. (7) This can be hard not only on the person with depression but on the people who care about them, or rely on them, too.
While people can experience depression at any age, it's most common in young adults. According to a 2022 study, the median age at which people experience depression symptoms is 26. About 25% of people notice symptoms before age 17, while another 25% don't have symptoms until after age 34. (8) Gender also plays a role—women are twice as likely to experience depression as men. (9)

Do I Have Depression? How Is Depression Diagnosed?

Unlike most physical conditions, depression cannot be caught through blood work or lab tests. If someone is experiencing symptoms, the best way to get a diagnosis for depression is by talking to a doctor. Many doctors conduct depression screenings during annual physicals, postpartum visits, and chronic illness check-ups.
During a screening, a doctor will ask about family history, lifestyle, and mood. They may also order tests to confirm that symptoms are not caused by a physical condition. Generally, a patient must experience persistent sadness, loss of interest in things they enjoy, and at least three other symptoms to be diagnosed. (10)

What Is the Best Treatment for Depression?

When depression is diagnosed, doctors often recommend counseling as treatment. Therapists work with patients to manage stress, improve mood, and change overall thinking and behavioral patterns. In some cases, the patient may also be prescribed antidepressants. There are five main types of antidepressants:

Antidepressants work by increasing the levels of certain neurotransmitters in the brain, such as serotonin or norepinephrine. (11) They've achieved noticeable results in up to 60% of patients (12).

How to Cope With a Depression Diagnosis

Even if a person already suspects they're depressed, receiving an official diagnosis can feel daunting. It's common to experience a wide range of emotions, including:

  • Confusion
  • Shame
  • Anger
  • Guilt
  • Relief
  • Hope

Thankfully, between 80% to 90% of patients report symptom relief after receiving depression treatment. (13)

How to Help Someone With Depression

Although depression medication and counseling can be effective, many people with depression don't seek treatment 14. It's common for depression patients to blame themselves and withdraw. If someone is showing common symptoms of depression, there are a few ways to help them:

  • Have a conversation: Ask loved ones how they're feeling and let them know they have support.
  • Encourage treatment: Remind them that depression is a mood disorder, not a personality flaw, and that treatment may help.
  • Ask questions: See if they need anything specific.

Mental health assistance is key to overcoming depression. Many insurance companies cover treatment, so contact providers directly and ask about the options available.