The modern understanding of bipolar disorder is that it is primarily a biological illness with a heritable component. However, the exact causes of the condition - the things that set it in motion - are not definitively understood at this time. Whether the culprit is a malfunctioning neurotransmitter, a misaligned hormone system, an infectious agent, or an elevated antibody, the cause of bipolar disorder is the subject of widespread debate. While the cause or causes are up for discussion, it does appear likely that bipolar disorders result from an imbalance in the body's regulatory systems. The fact that bipolar disorders can typically be controlled by medication strongly suggests that they result from biochemical imbalances, most likely occurring in the brain. These imbalances are an indication that a regulatory system has become impaired in some way. Although it is thought that bipolar disorders are the result of the body's chemical disturbances, there is no biological diagnostic test to reliably identify bipolar disorder. Research in multiple medical disciplines is ongoing in an effort to understand more about bipolar disorders, find dependable diagnostic tools, and develop practical treatments.
Bipolar Causes: Diathesis-Stress Hypothesis
Like any other large scale and public human endeavor, the process of medical research is influenced by prevailing ideas and cultural currents. Medical research into the causes and origins of bipolar illness has been influenced by a prevailing idea known as the Diathesis-Stress Hypothesis which has been applied to many medical disorders. The basic diathesis-stress hypothesis proposes that people have predispositions and vulnerabilities for illnesses (known as diatheses). Some people have more of these susceptibilities than others, for varying reasons having to do with their genetics, biology and experience. Merely possessing a vulnerability for an illness alone is not enough to trigger that illness into action, however. Instead, people's vulnerabilities must interact with life stresses to prompt the onset of the illness. The greater a person's inherent propensity for developing an illness, the less stress is necessary to get the illness started. Conversely, where there is a smaller susceptibility for developing an illness, a greater amount of stress is required to produce the illness. Until this critical amount of stress is reached (however much or little of it is necessary in a given case) people cannot be said to have an illness, and their vulnerabilities are said to be "latent" (hidden).
The current thinking regarding the causes of bipolar disorder is essentially that bipolar disorder results when bipolar diatheses (hidden and unexpressed vulnerabilities) meet a source of sufficient stress necessary to activate them and cause the disease to start.
It's not quite as simple as the preceding statement may sound, of course. For instance, various sources of stress may combine to produce the releasing effect, including stressors which are psychological, social, and biological. For example, trauma, family stress, depression, viruses, and birth complications can be forces powerful enough to provoke the weakness towards bipolar illness. Other stresses that can add to vulnerability include sleep deprivation ( known to trigger mania), Seasonal Affective Disorder (SAD), alcoholism, drug abuse, imbalances in brain chemicals, hormones, infections, and premenstrual syndrome in women. Although the relationship between menstrual cycles and bipolar disorder is still unclear, this connection may contribute to the fact that more women experience depressive episodes than men. It is important to remember that the events or stresses that set an illness in motion can be very different from those that keep it going.
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Evidence that bipolar disorder is biological in nature comes from a wide range of studies in the genetics, neurochemistry, endocrinology and immunology fields.
Bipolar Causes: Genetics
Observations of family histories have long confirmed that bipolar disorders are inherited genetically. Below are some recent statistics showing the percentage of people who typically go on to develop a bipolar disorder given their particular relationship with a relative who is similarly affected.
In all, about 80-90% of people diagnosed with bipolar disorder have a family history of either bipolar or major depression. However, this does not necessarily mean that someone with such a family history will inherit the genetic predisposition of the disease, or if they do, that their predisposition will eventually develop into symptoms.
Areas of chromosomes 4, 12, 18, 21, 22, and X have been linked to various aspects of bipolar disorder supporting the general notion that the illness is caused by multiple genes. At the present time, however, there are no genes that have been identified as definitively responsible for causing bipolar disorder. It is likely that genes that control bipolar disorders will be identified some time in the future. However, in accordance with the diathesis-stress hypothesis, the mere presence of particular genes alone is not expected to be sufficient to set the disorder in motion. How the environment reacts with genes is an important component of how bipolar develops and is then sustained.