Research indicates that depression is, at least in part, inherited. Thus far, however, no studies have isolated the specific genes responsible for depression.
2. Brain Chemistry Imbalance
Depression is believed to be caused by an imbalance in the neurotransmitters which are involved in mood regulation. Neurotransmitters are chemical substances which help different areas of the brain communicate with each other. When certain neurotransmitters are in short supply, this may lead to the symptoms we recognize as clinical depression.
3. Female Sex Hormones
It has been widely documented that women suffer from major depression about twice as often as men. Because the incidence of depressive disorders peaks during women's reproductive years, it is believed that hormonal risk factors may be to blame. Women are especially prone to depressive disorders during times when their hormones are in flux, such as around the time of their menstrual period, childbirth and perimenopause. In addition, a woman's depression risk declines after she goes through menopause.
4. Circadian Rhythm Disturbance
One type of depression, called seasonal affective disorder, is believed to be caused a disturbance in the normal circadian rhythm of the body. Light entering the eye influences this rhythm, and, during the shorter days of winter, when people may spend limited time outdoors, this rhythm may become disrupted.
5. Poor Nutrition
A poor diet can contribute to depression in several ways. A variety of vitamin and mineral deficiencies are known to cause symptoms of depression. Researchers have also found that diets either low in omega-3 fatty acids or with an imbalanced ratio of omega-6 to omega-3 are associated with increased rates of depression. In addition, diets high in sugar have been associated with depression.
6. Medical Illnesses
Illness is related to depression in two ways. The stress of having a chronic illness may trigger an episode of major depression. In addition, certain illnesses -- for example, thyroid disorders, Addison's disease and liver disease -- can cause depression symptoms.
7. Drugs, Both Legal and Illegal
Several prescription drugs have been reported to cause symptoms of depression. In addition, a variety of drugs of abuse have been associated with depression symptoms.
8. Stressful Life Events
Stressful life events, which overwhelm a person's ability to cope, may be a cause of depression. Scientists have theorized that the high levels of the hormone cortisol, which are secreted during periods of stress, may somehow induce depression by affecting the neurotransmitter serotonin.
9. Grief and Loss
Although grief is a normal response to death and loss, the extreme stress associated with grief can trigger an episode of major depression.
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Moore, David P. and James W. Jefferson, eds. Handbook of Medical Psychiatry 2nd ed. Philadelphia: Mosby Elsevier, 2004.
Piccinelli, Marco and Greg Wilkinson. "Gender differences in depression." British Journal of Psychiatry 177 (2000): 486-492.