Genetic or hereditary factors contribute to the risk of bipolar disorder. Studies have demonstrated that the prevalence of bipolar disorder is higher among the parents of bipolar patients. However, hereditary factors are not the only cause of bipolar disorder.

Studies of identical twins suggest that both genes and other factors play a role in the cause of bipolar disorder. Identical twins share all the same genes. If bipolar disorder were caused entirely by genes, then the identical twin of a patient with the illness would always develop the illness, which is not the case.

However, if one twin has bipolar disorder, the other twin is more likely to develop the illness. There is a slightly greater risk of developing the disease among siblings as well.

Other than genetic factors, there are no other known risk factors for bipolar disorder. Lack of sleep may predispose individual patients to a recurrence of symptoms. Psychological and hormonal factors are suspected of playing a role as well. Some medications and illnesses cause symptoms that mimic bipolar disorder. It is important for these to be ruled out before a diagnosis of bipolar disorder is made.