The exact cause of bipolar disorder is unknown. It is estimated that there is a 71 percent chance of bipolar disorder passing from parent to child. If one identical twin has bipolar disorder, there is a 40 to 70 percent chance that the other will develop bipolar disorder in their lifetime. MRI studies have shown that certain parts of the brain responsible for mood regulation are larger in bipolar patients. It is also possible for bipolar disorder to develop after a stroke, brain injury, or certain infections.