The cause of bipolar disorder is unclear, but hereditary, biological, and psychological factors may play a part. For example, the incidence of bipolar disorder among relatives of affected patients is higher than in the general population and highest among maternal relatives. The closer the relationship, the greater the susceptibility. Children with one affected parent have a 25% chance of developing bipolar disorder; children with two affected parents, a 50% chance. The incidence of this illness in siblings is 20% to 25%; in identical twins, the incidence is 66% to 96%.

While the exact cause of bipolar disorder (manic depression) has yet to be determined, there is a great deal of scientific evidence that indicates a chemical imbalance in the brain. As for what causes this imbalance, there are a number of viable theories, which center on a combination of heredity and environmental triggers. Presented here is information relevant to these theories.

According to the U.S. government’s National Institute of Mental Health (NIMH), “There is no single cause for bipolar disorder — rather, many factors act together to produce the illness.” “Because bipolar disorder tends to run in families, researchers have been searching for specific genes passed down through generations that may increase a person’s chance of developing the illness.” “In addition, findings from gene research suggest that bipolar disorder, like other mental illnesses, does not occur because of a single gene.”

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.

Brain-imaging studies are helping scientists learn what goes wrong in the brain to produce bipolar disorder and other mental illnesses. New brain-imaging techniques allow researchers to take pictures of the living brain at work, to examine its structure and activity, without the need for surgery or other invasive procedures. These techniques include magnetic resonance imaging (MRI), positron emission tomography (PET), and functional magnetic resonance imaging (fMRI). There is evidence from imaging studies that the brains of people with bipolar disorder may differ from the brains of healthy individuals.

Genes are the blueprint for all cells and their contents. Scientists thus believe that changes to genes can lead to faulty proteins being produced within brain cells. These faulty proteins may then result in bipolar disorder. But genes are not the whole story. Studies of identical twins, who share all the same genes, indicate that both genes and other factors play a role in bipolar disorder. If bipolar disorder were caused entirely by genes, then the identical twin of someone with the illness would always develop the illness, and research has shown that this is not the case. But if one twin has bipolar disorder, the other twin is more likely to develop the illness than is another sibling.

Bipolar disorder involves periods of elevated mood, or mania. Usually—but not alwaysthe disorder also involves periods of depression. In a typical case, a person with bipolar disorder cycles between these two extremes—experiencing recurrent episodes of both elevated and depressed mood, often with symptom-free stretches in between.