Many factors contribute to clinical depression - stress, genetic predisposition, psychological factors, imbalances of neurotransmitters in the brain.

Depression can occur for no apparent reason. A depressed person may not be able to think of any reason for feeling down, and may feel like a failure for not being able to snap out of it. But clinical depression is a medical illness, not a personal failure, and there are many factors that can contribute to it.

Depression may be triggered by stress. Sometimes the cause of stress is obvious - the loss of a loved one, a divorce or separation, chronic illness, financial troubles, abuse or any major trauma.

Other times stress may be caused by a ‘transitional’ period in one’s life, e.g. adolescence, new parent, mid-life, retirement. Having problems dealing with significant changes in one's life is known as an adjustment disorder - a form of depression that usually resolves without treatment. For some people, an adjustment disorder can spiral into major depression.

Normal bereavement is not major depression, even though many of the symptoms are the same. Though one cannot put a time limit on grief, if it seems to be very long lasting or the person is unable to perform day-to-day activities, it is possible that grief has triggered major depression.

Loss of a parent early in life puts one at a much higher risk of developing depression.

Psychological Factors:

Low-self esteem, poor coping skills, lack of social support and loneliness caused by isolation increase the risk of depression. (Depression can also cause low self-esteem and cause one to isolate themselves so it may be difficult to tell which came first)

A depressed person may blame his or her depression on a particular problem. The person may believe all would be fine if the problem was solved. But it is more likely stress caused by an overblown reaction to the problem not only triggered the depression but also has prolonged it. Prolonged stress, no matter what the cause, can trigger clinical depression. In turn, depression affects thinking and attitude and the problem may become magnified. Psychotherapy can help one deal with one's issues, recognize distorted, destructive thinking patterns, and speed recovery from depression.

Genetic Factors:

A tendency towards depression tends to run in families, but depression can affect anyone.

Other Chronic Medical Conditions:

A heart attack, stroke, cancer, Parkinson's disease and other illness may trigger depression for both physical and psychological reasons.

Medications:

Some medications taken for other conditions, such as high blood pressure, can cause depression.

Chemical Imbalances:

People suffering from clinical depression usually have low levels of certain neurotransmitters, especially serotonin. Neurotransmitters are chemical messengers in the brain that transmits electrical signals between nerve cells. There are many other neurotransmitters, but serotonin, dopamine, and norepinephrine are most implicated in affecting mood.

Neurotransmitters in the brain also decrease as a result of stress and sadness. So are low levels of neurotransmitters the cause of depression or simply the result of depression - or does it even matter as long as treatment corrects the imbalance.

While the focus has been on depression being caused by a chemical imbalance in recent years, the 'chemical imbalance' explanation may be a little too simplistic. It is interesting to note that while an increase in neurotransmitters happens almost immediately after going on antidepressant medication, the effects on mood take usually take several weeks to develop. Antidepressants may work through mechanisms other than or in addition to increasing levels of neurotransmitters.

Whether through medication or psychotherapy or exercise, relief of depression corresponds with an increase in the levels of neurotransmitter in the brain.

Brain imaging shows that there are differences in the amount of neural activity, increased in some areas and decreased in other areas, in the brains of people who are depressed. When the depression goes away, this activity returns to normal. However, there may be some differences in how the brains of people who have depression function, even when they are not experiencing depression.

In a response to stress, the body releases extra cortisol and other stress hormones. About half of people suffering from depression have elevated levels of cortisol.

Hormonal changes can trigger depression in some people - e.g. the hormonal changes after giving birth.

Structural Differences of the Brain:

It is believed that structural anomalies of the brain, and not just a chemical imbalance in the brain, may play a role in depression. With brain imaging, it can be seen that the brains of some depressed people show structural differences from brain of non-depressed people. These differences are thought to be largely genetic, although environmental factors can affect the developing brain during childhood.

Traumatic events and extreme stress in childhood (such as the loss of a parent or abuse) can physically alter brain circuitry and make one susceptible to depressive illnesses later on.

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