Depression can have several different underlying causes. It is likely caused by some combination of environmental, genetic, psychological and biochemical factors. In some cases, depression can be triggered by an event such as a death or loss of job while other causes are more systemic such as age or heredity.
Also, there are some Hypothesized Adaptive Evolutionary Theories of Depression that point to some evolutionary theories on why we get depressed.
And finally, some Psychological Perspectives on the Etiology of Depression seek to explain depression from the standpoint of unexpressed and repressed anger.
What are the Causes of Depression?
There are many explanations for what causes people to become depressed, and each person reacts to these potential causes differently. Two people who experience the same traumatic event may internalize the event differently, leading one to become depressed and one to not exhibit any symptoms at all. Or, both individuals may become depressed, but one may show symptoms of anxiety and anger while the other experiences symptoms of insomnia and mental fatigue.
Certain stressful or traumatic situations that occur outside one's self (i.e. occurring from the environment) can trigger a depressive episode. For example, the loss of a loved one, a divorce, or a trauma such as a serious accident can precipitate a depressive episode. Severe early neglect can result in persistent abnormalities in corticotropin releasing factor (CRF), which can result in a vulnerability to depression.1 Depression can also be compounded by the stress of responsibilities such as caring for children and aging parents. Other life challenges such as poverty, relationship strains, or poverty may act as triggers to depression in certain people. Studies suggest that psychosocial events (stressors) may play a more significant role in the precipitation of the first or second episodes of Major Depressive Disorder and may play less of a role in the onset of subsequent episodes.
Inadequate nutrition can also lead to depression. Making sure one has enough of the nutrients necessary and very few of the toxic substances which can cause it, are imperative to treating depression.
Due to technology, there exists a growing separateness and isolation. Our leisure time is spent in front of a TV or a computer screen, often alone. In past times, many activities were done in the company of a group and not in isolation.13
Genetic risk for depression has been linked with multiple genes interacting with environmental or other factors.2 Genetic studies (primarily derived from population, family, twin, and adoptive studies) suggest that certain depressive disorders may have a genetic loading, especially bipolar and recurrent unipolar depression. In both human and animal studies, exposure to two general classes of stress may result in the neurotransmitter abnormalities underlying depression; this is especially true in those with a genetic susceptibility to depression. These classes of stress include experiences of loss and exposure to highly stressful situations where no avoidance, escape, or coping is available (resulting in an eventual state of learned helplessness).3 Also worth noting is that Major Depressive Disorder is 1.5 to 3 times more common among first-degree biological relatives of person with this disorder than among the general population.4
Neuro-researchers have hypothed that many of the primary symptoms of clinical depression are caused by a dysregulation of certain neurotransmitters like norepinephrine and serotonin.5 (Individuals with depression caused by hormonal imbalance are the most likely to positively respond to antidepressant treatments as these seek to realign the neurotransmitter levels in the brain.) Women may be more prone to depression due to significant hormonal changes that take place with menstruation, child birth (often "postpartum depression"), and menopause. Researchers are studying the interaction of hormonal processes and its impact on depression.
Aging and Illnesses
Individuals with chronic or severe general medical conditions are at increased risk to develop major depressive disorder. Certain medical illnesses can result in systemic biochemical and hormonal changes that affect the central nervous system. Up to 20%-25% of individuals with certain general medical conditions (e.g., diabetes, thyroid disease, viral infections, myocardial infarction, carcinomas, stroke) will develop major depressive disorder during the course of their general medical condition. Older adults are more likely to have severe medical conditions such as heart disease and cancer. Additionally, primary neurological disease commonly found in elderly people (e.g., Parkinson's disease, Alzheimer's disease) may affect or destroy brain areas responsible for affect regulation.6 These conditions as well as their corresponding medications can contribute to depression in older adults. A majority of older adults with depression can improve when they receive treatment such as antidepressants, psychotherapy, or some combination of both.
Treatment Resistant Depression
It is important to accurately identify the causes of your individual depression as best as possible. Many people begin treatment to alter their neurotransmitter levels (usually with antidepressant medication) but without addressing the other possible causes, their treatment does not work. This is known as treatment resistant depression; when treatments do not seem to work.
If your symptoms have continued despite your attempt to cure depression with many treatment modalities, you may have this condition. Be sure to find a practitioner who will help you identify as many underlying causes and possible contributors to your depression, and will help you find the modalities that will help you treat it!
1 Nemeroff, C. B.(1997). The role of early adverse life events in the etiology of depression and post-traumatic stress disorder: Focus on CRF. Annals of the New York Academy of Sciences, 821,194-207.
2 Tsuang, M. T., Bar, J. L., Stone, W. S., & Farasone, S. V. (2004). Gene-environment interactions in mental disorders. World Psychiatry, 3(2), 73-83.
3 Weiss, J.M. et al. (1975). Psychosomatic Medicine, 37, 522-533.
4 American Psychiatric Association. (2000). Diagnostic and statistical manual of mental disorders, fourth edition, revised. Washington DC: Author.
5Preston, J. & Johnson, J. (1994). Clinical psychopharmacology made ridiculously simple. Miami, FL: MedMaster, Inc.^^
6 Preston, J. D, O'Neal, J. H., & Talaga, M. C. (2005). Handbook of clinical psychopharmacology for therapists, fourth edition. Oakland, CA: New Harbinger Publications, Inc.