What causes depression and other mood disorders?

Although depression is a complex illness with many factors playing a role in its development, current scientific evidence indicates that the fundamental problem is a hereditary and biological one. We are not sure of the exact nature of the problem, but it is clear that depression and bipolar illness are due to defects in brain biochemistry and structure, and perhaps the workings of the glandular and immune systems as well.

In this sense, they are not mental illnesses (sicknesses of the mind) but physical illnesses of the brain. With advances in our understanding of the biochemical problems involved in mood disorders, the term mental illness will eventually be discarded.

Scientific evidence for the genetic, biochemical, and physical basis of depression comes from several sources:

Mood disordersIdentical-Twin Studies

  • These show that when identical twins (who have the same genetic makeup) are reared apart and one twin has clinical depression, the other twin is more likely to develop depression than would be expected by chance alone.

Adoption Studies

  • Individuals with depressed parents or other relatives have been found to develop depression more frequently than would be expected by chance alone even if they are brought up by nondepressed adoptive parents.

Biochemical Studies

  • Depressed individuals have some similar abnormalities in brain biochemistry.

Neuroimaging Studies

  • Pictures of brain structure and metabolism of people with depression show some similarities. (Imaging techniques may eventually help doctors make a diagnosis in individual patients.)

Medication Response Studies

  • The majority of people with symptoms of depression respond to medications believed to correct specific brain biochemical imbalances.

This is not to say that the environment an individual was raised in or later unfortunate life circumstances play no role in mood disorders. The current thinking is that the genetic and biochemical dysfunctions in depression are triggered by life events, including problems in the family in which a depressed individual grew up. If a person with a genetic vulnerability to depression is raised by abusive alcoholic parents, for instance, she is more likely to develop depression than is a genetically vulnerabe individual brought up in a stable home. Someone suffering from depression can usually come up with good reasons for his or her emotional pain. The person might be able to point to a rotten childhood or current problems to explain his troubled mood. But these reasons alone often do not explain why someone would go from feeling depressed to developing the many symptoms characteristic of depressive illness. Many people go through profoundly painful traumas of all kinds and do not end up with symptoms of depressive illness.

The prospective patient needs to keep in mind that once a depressive illness takes hold, it can develop a life of its own, regardless of the events that triggered it. Even when depressive illness has clearly been set off by negative events, medical treatment may be needed to help resolve it.

A few studies have suggested that at least some cases of serious depression could be caused by viruses, including a virus called the Borna virus. A large percentage of people with bipolar disorder were born in the winter at the height of cold and flu season. The antiviral drug amantadine/Symmetrel has reportedly helped some patients with clinical depression. In addition to being an antiviral drug, however, amantadine also enhances the effectiveness of a neurotransmitter that may be implicated in depression, so its effectiveness could have as much to do with this action as with its antiviral action.