We have all asked this question at some time. I've heard explanations ranging from a shortage of lithium in the brain to dog bites in childhood. Obviously, there is a great deal of misinformation to be had.
To add to the confusion, scientific research continues to publish new information and theories. A 2000 study in the American Journal of Psychiatry reported "in those with bipolar disorder, two major areas of the brain contain 30 percent more cells that send signals to other brain cells." This report theorizes that "the extra signal-sending cells may lead to a kind of overstimulation, which makes sense considering the symptoms of bipolar disorder."
But has anyone found the true cause of bipolar disorder? It would be wonderful to say that X or Y was the cause, but the answer is not that simple. Most scientists believe that mental illnesses are caused by a combination of several factors working together. In bipolar disorder, these factors are usually divided into biological and psychological causes. In plain English, the main reasons mental illness develops are physical (biological) and environmental.
When talking about biological causes, the first issue is whether bipolar disorder can be inherited. This question has been researched through multiple family, adoption and twin studies. In families of persons with bipolar disorder, first-degree relatives (parents, children, siblings) are more likely to have a mood disorder than the relatives of those who do not have bipolar disorder. Studies of twins indicate that if one twin has a mood disorder, an identical twin is about three times more likely than a fraternal twin to have a mood disorder as well.
In bipolar disorder specifically, the concordance rate (when both twins have the disorder) is 80 percent for identical twins, as compared to only 16 percent for fraternal twins. (Identical twins occur when one fertilized egg splits in two, so they share the same genetic material; fraternal twins come from separate fertilized eggs, so the mixtures of genetic material are different.) There is overwhelming evidence that bipolar disorder can be inherited and that there is a genetic vulnerability to developing the illness.
However, exactly what is inherited? The neurotransmitter system has received a great deal of attention as a possible cause of bipolar disorder. Researchers have known for decades that a link exists between neurotransmitters and mood disorders, because drugs which alter these transmitters also relieve mood disorders.
- Some studies suggest that a low or high level of a specific neurotransmitter such as serotonin, norepinephrine or dopamine is the cause.
- Other studies indicate that an imbalance of these substances is the problem, i.e., that a specific level of a neurotransmitter is not as important as its amount in relation to the other neurotransmitters.
- Still other studies have found evidence that a change in the sensitivity of the receptors on nerve cells may be the issue.
For mental, emotional and environmental issues, stressful life events are thought to be the main element in the development of bipolar disorder. These can range from a death in the family to the loss of a job, from the birth of a child to a move. It can be pretty much anything, but it cannot be precisely defined, since one person's stress may be another person's piece of cake.
With that in mind, research has found that stressful life events can lead to the onset of symptoms in bipolar disorder. However, once the disorder is triggered and progresses, "it seems to develop a life of its own." Once the cycle begins, psychological and/or biological processes take over and keep the illness active.
Putting it all together
When we look for the cause of bipolar disorder, the best explanation according to the research available at this time is what is termed the "Diathesis-Stress Model." The word diathesis means, in simplified terms, a physical condition that make a person more than usually susceptible to certain diseases. Thus the Diathesis-Stress Model says that each person inherits certain physical vulnerabilities to problems that may or may not appear depending on what stresses occur in his or her life. Durand and Barlow define this model as a theory "that both an inherited tendency and specific stressful conditions are required to produce a disorder."
So the bottom line, according to today's thinking, is that if you are manic depressive, you were born with the possibility of developing this disorder, and something in your life set it off. But scientists could refine that theory tomorrow. The one sure thing is, they won't give up looking for answers.
Bernstein, D. A., Clarke-Stewart, A., Penner, L. A., Roy, E. J., & Wickens, C. D. (2000). Psychology (5th ed.). Boston, MA: Houghton Mifflin Company.
Davis, S. F., & Palladino, J. J. (2000). Psychology (3rd ed.). Upper Saddle River, NJ: Prentice-Hall, Inc.
Durand, V. M. & Barlow, D. H. (2000). Abnormal Psychology: An Introduction. Scarborough, Ontario: Wadsworth.
The University of Michigan. (October, 2000). Evidence of Brain Chemistry Abnormalities in Bipolar Disorder. Accessed October 30, 2000.