Causes of Depression:

The interrelated causes of depression

Depression is a common response to life's disappointments, trauma, and loss. We may feel down in the dumps after a romantic break-up or when we don't get a job offer we'd hoped for. However, these episodes of depression are typically brief in duration. As we work through our feelings of sadness and discouragement, they gradually disappear. But in the case of chronic depression, the symptoms don't dissipate, but persist and even get worse.

While the causes of chronic depression are not known with certainty, many factors are believed to be involved. These causes are interrelated, interacting and impacting each other. Biological factors such as heredity and chemical imbalances in the brain are believed to play a role. But equally important are psychological and social factors. These include levels of stress in your life, the quality of your support system, your way of looking at the world, your coping style, your early life experiences, and lifestyle choices.

Chemical imbalances and depression

Neurotransmitters are chemical messengers in the brain. They play a role in everything the brain regulates, including mood and emotions.  Lower levels of three neurotransmitters—serotonin, norepinephrine, and dopamine—have been found in people suffering from depression. This discovery is behind the “chemical imbalance” theory that depression is caused by deficiencies in one or all of these neurotransmitters in the brain. However, it is unclear whether neurotransmitter imbalances give rise to depression or whether the depressive illness itself causes the imbalance. Furthermore, research has shown that not all depressed people have decreased neurotransmitter activity. Some studies have even found increased levels of norepinephrine in depressed patients.not all depressed people have decreased neurotransmitter activity. Some studies have even found increased levels of norepinephrine in depressed patients.

Biological causes of depression

Aside from the role neurotransmitter imbalances may play, other biological factors involved in the development of depression include genes and hormones.

Genetics and heredity

Depression runs in families, but it is difficult to decipher how much of this is due to genetics and how much is due to environmental influences. However, twin studies provide information about the role of genetics. Identical twins have identical genes, so if one twin develops depression and the other doesn’t, it’s clear that environmental factors are behind the difference. Twin studies suggest that there is a moderate genetic contribution to depression. Genetics appears to play a bigger role in severe, recurrent depression. The genetic influence on more mild forms of depression such as dysthymia is much lower.

Hormonal imbalances

Women are twice as likely as men to experience depression. Some researchers believe that hormonal factors are involved in this increased risk. Cortisol—a hormone the body produces as part of the stress response—has also been found to be elevated in some depressed people. However, like neurotransmitter imbalances, it is unclear whether the elevated stress hormones cause or are caused by depression.

Social and psychological causes of depression

Biology is not destiny. If you have a close relative that suffers from depression, statistically you’re at a higher risk of developing depression yourself. But depression is by no means an inescapable fate. Just as there are risk factors for depression, there are also protective factors. A strong support system, a rewarding work or family life, a sense of self-confidence and control over your own destiny, an optimistic outlook—all these things can keep depression at bay, even if you have a genetic predisposition for it. On the other hand, certain social and psychological factors can also cause depression, particularly in those already vulnerable biologically.

• Loneliness and isolation – Those who feel lonely and isolated are at a high risk of becoming depressed. Depression rates are higher among people who live alone and have a limited social network. But even people surrounded by family and friends are at risk for depression if those relationships are unsupportive.
• Stressful life events – Major life changes and stressful events take a toll on mental health. Depression is common after the loss or threatened loss of a loved one through divorce, death, or some other kind of separation. Other stressors such as moving, unemployment, retirement, or   illness can also trigger depression.
• Childhood trauma and loss – Loss, adversity, and stress early in life create a vulnerability to depression. Children who lose parents through death or separation are particularly at risk. Other childhood stressors that can contribute to depression later on include physical or sexual abuse, family turmoil and strife, and parents who suffer from mental illness, including depression.
• Personality traits - Introversion, low self-esteem, pessimism, and dependence are personality traits that can contribute to depression. Attribution style is another important factor. People who attribute negative events to internal (It’s my fault), stable (It’s never going to change), and global (It will always be like this) causes are prone to depression. Feelings of helplessness and hopelessness are particularly damaging. People who feel that they have no control over negative events are very likely to become depressed.

Medical causes of depression

A variety of medical conditions can affect mood and lead to depression. Illnesses that affect that brain—such as Parkinson’s disease, multiple sclerosis, and Alzheimer’s disease—can cause depression through the disease process itself. Depression can also be a symptom of a medical condition. For instance, depression often occurs in diseases that affect the immune system or the body’s hormones. Hypothyroidism, a condition caused by low levels of thyroid hormone, so commonly contributes to depressed mood and fatigue that it should always be ruled out before depression is diagnosed.

Medical Conditions that Can Cause or Worsen Depression
General Medical Conditions	Infectious Diseases
Diabetes Heart disease Sleep disorders Cancer Lupus	HIV Syphilis Lyme Disease Chronic Fatigue Syndrome Mononucleosis
Neurological Diseases	Endocrine Disorders
Stroke Epilepsy Alzheimer’s disease Parkinson's disease Multiple sclerosis	Hypothyroidism Hyperthyroidism Addison's disease Cushing's disease Hyperparathyroidism

Health problems that cause chronic pain or disability can also trigger depression. The risk of depression is highest when the physical problems cause major lifestyle upset. The avid golfer who can no longer play because of arthritis pain, the independent senior citizen forced to give up her driver’s license, the young sports buff confined to a wheelchair—all must deal with new limitations. Those that aren’t able to adjust are likely to become depressed.

But it’s not just medical conditions that poses a problem. Sometimes the cure for a disease can cause depression. In fact, many medications list depression as a side effect—including antidepressants. The risk of medication-induced depression is highest when these drugs are taken over long periods of time.

Lifestyle factors

Unhealthy lifestyle choices such as a sedentary daily routine, a diet full of empty calories, and excessive alcohol and drug use can trigger a depressive episode or make an existing depression worse.

Lack of exercise

Exercise can be a huge mood booster. In fact, studies show that the “exercise cure” is at least as effective, if not more so, then antidepressant treatment. Researchers have found that exercising as little as three times a week can lift the symptoms of depression. Daily walks of thirty minutes or more are even more effective. Conversely, a sedentary lifestyle can put you at risk for developing depression.

A poor diet

Poor eating habits can wreak havoc on your mood. Junk food and sugary snacks can cause rapid changes in blood sugar, resulting in a temporary “high” followed by a crash. The same is true of caffeinated beverages. And if you aren’t eating enough produce, lean protein, and complex carbohydrates, you probably aren’t getting enough of the nutrients your mind and body need. In fact, certain vitamin and mineral deficiencies have been linked to depression. Research has shown that deficiencies in folic acid and vitamin B-12 can cause symptoms of depression. Vitamin C deficiency, found in people who don't eat fruits and vegetables, can also cause fatigue and sadness.

Amino acid deficiencies can also result in depression. SAMe is an important amino acid that has been found to be low in some depressed people. SAMe supplements are sometimes used to treat depression. Tryptophan is another amino acid that is sometimes low in depressed people. A supplement called 5-HTP helps elevate tryptophan levels and has been shown to lift mood. Finally, low fat diets that don't include enough omega-3 fatty acids can also lead to depression. If you'd like to get more omega-3 in your diet, you can eat more fish, eggs, and flax seed, or you can pop a fish oil supplement.

Substance abuse

Although some people abuse alcohol and drugs in an effort to self-medicate and ease their symptoms of depression, it is now believed that substance abuse itself causes depression. Alcohol—as a depressant that slows down brain activity—is particularly dangerous. Excessive marijuana use can also lead to depression. Finally, although “uppers” such as amphetamines and cocaine initially stimulate the nervous system, when the effect wears off depression is common.

Related links for causes of depression 

General information about depression causes

Causes of Depression – In-depth discussion of the causes of depression, including brain function, hormones, genetics, childhood difficulties, and stress. (All About Depression)

What Causes Depression? - Explore the impact of psychological makeup, heredity, biochemical factors, and hormonal influences on depression. (Yahoo! Health)

What Causes Depression? - Article describes the many causes of depression and gives advice on taking back control of your life. (Psychology Today)

Genetic causes of depression

Depression Gene May Weaken Mood-Regulating Circuit – Read about the research findings regarding a gene that may increase susceptibility to depression and anxiety. (National Institute of Mental Health)

Gene Linked to Both Alcoholism and Depression – Learn about a gene that researchers believe to be linked to both alcoholism and depression. (Washington University in St. Louis, School of Medicine)

Genetic Causes of Depression – Covers the influence of heredity on depression, including evidence from twin studies. (All About Depression).

Chemical imbalances and depression

The Brain and Depression – Describes how social and psychological factors affect brain chemistry, rather than just the other way around. (Psychology Today)

Some Drugs Work to Treat Depression, But It Isn't Clear How – Reviews research challenging the chemical imbalance theory of depression. (Wall Street Journal)

Medical causes of depression

Medicines That Can Cause Mood Disorders – An extensive list detailing some of the medicines used to treat other conditions that can have side effects of depression or other mood disorders. (Cleveland Clinic Health System)

Cancer and Depression – Learn about the risk of depression in cancer patients, and how to treat the problem. (National Cancer Institute)

Depression and Stroke - Article on the causes of depression in stroke victims and what treatments are available. (Blue Cross and Blue Shield of Massachusetts)

Depression and Alzheimer's Disease – Describes the connection between Alzheimer’s disease and depression and what can be done to help. (American Academy of Family Physicians)

Melinda Smith and Jaelline Jaffe, Ph.D. contributed to this article. Reviewed by Jeanne Segal, Ph.D., Last modified on: 11/07/07.