Causes of Major Depressive Disorder

Scientists have not yet determined the root cause of major depression. However, there is strong evidence there may be several contributors to the illness. Psychological, biological and environmental factors may all contribute to its development. Whatever the specific causes, research has firmly established that major depression is a biological brain disorder.
Serotonin, norepinephrine and dopamine are three neurotransmitters (chemical messengers that transmit electrical signals between brain cells) thought to be involved with major depression. Several theories attempting to explain depression are based on an imbalance of these chemical messengers. It is thought that most antidepressant medications work by increasing the availability of neurotransmitters or by changing the sensitivity of the receptors for these chemicals.
Scientists have also found evidence of a genetic predisposition to major depression. There is an increased risk for developing depression when there is a family history of the illness. Not everyone with a genetic predisposition develops depression, but some people probably have a biological make-up that leaves them particularly vulnerable to developing depression. Life events, such as the death of a loved one, chronic stress, and alcohol and drug abuse, may trigger episodes of depression. Some illnesses such as heart disease and cancer and some medications may also trigger a depressive episode. Often, however, depressive episodes occur spontaneously and are not triggered by a life crisis or physical illness.