There is evidence that Bipolar Disorder is genetic. An individual with this disorder is more likely than not to have a close relative with the same disorder, and there is a 65% chance that both identical twins have the disorder, compared to only 14% of fraternal twins. The physiological basis of the disorder is supported by neuroimaging studies that reveal that the two sides of the brain are different. In addition, neurochemical imbalances play a significant role in the cause of the disorder, pointing the way to its most effective treatment. Excess dopamine and a disregulation of norepinephrine are the probable causes targeted by medication.

Environment also is thought to play a causal role. Some studies of identical twins show that one twin has the condition while the other doesn't — which means genes alone aren't responsible for bipolar disorder. Environmental causes may include problems with self-esteem, significant loss or high stress.