Bipolar disorder is a difficult illness to manage and to treat. Many who have it may ask themselves, "Why me? What caused all this?" There are great disagreements as to the causes of bipolar disorder. They all tend to go back to the old nature/nurture controversy. In other words, does a thing happen to a person because of who he or she is, or because of the environment he or she grew up in?
The nature side of bipolar disorder causes has always been seen in family histories. This, however, can be misleading. Families often pass behaviors on from one generation to the next, regardless of whether family members are natural relatives or adopted ones.
The scientific concept of correlation without causation may account for shared histories of bipolar disorder in biologically unrelated siblings. This concept is easy to grasp. For example, a man could state that all summer, every time he got a sunburn he ate fish. So, did the sunburn cause the man to eat fish? No, but the act of fishing both caused the man's skin to burn and allowed him to catch a fish, which he then ate. In a similar way, bipolar disorder can occur in families without anything in one family member's bipolar disorder causing the bipolar disorder of another.
Also, for whatever reason, people with bipolar disorder are often drawn to each other. In this case it is unclear whether the families formed come together because of their shared genetically similar predisposition towards bipolar disorder, or whether some members of the families are genetically more prone to bipolar disorder but the illness of some other members of the family becomes exaggerated more than it would in another environment.
Research into the genetic causes of bipolar disorder is often done using twin studies. It is assumed that twins will have environments that are as close as is possible. Identical twins are used to show the effects of genetics, since they will share the same genetic materials. Fraternal twins are used as a control group. While these twins share nearly identical environments with their twins, the fraternal twins have less genetic material in common.
It has been shown through these twin studies, and other studies where identical twins are compared to adopted siblings, that there does seem to be a genetic basis for bipolar disorder. Only one percent of the population has bipolar disorder. Fraternal twins, who share some genetic information, are 20 percent more likely to have the disease if one has it. The percentage for identical twins is even higher, at around 60 to 80 percent chance of one having it if the other does.
Environmental causes of bipolar disorder are more difficult to assess. Bipolar disorder has been proven to have a chemical basis in the brain, but the chemical reactions can be caused by any number of factors. A history of losses early in life can be a contributing factor, as can any major source of stress. Physical illnesses such as cancer and others can lead to a depressive state, which is then often followed by mania.
Neither genetics nor environment can fully explain the causes of bipolar disorder. Research is constantly being undertaken in both areas. In the meantime, the nature/nurture controversy is just beginning to heat up.