Bipolar I Disorder is a life-long disease and runs in families but has a complex mode of inheritance. Family, twin and adoption studies suggest genetic factors. The concordance rate for monozygotic (identical) twins is 43%; whereas it is only 6% for dizygotic (nonidentical) twins. About half of all patients with Bipolar I Disorder have one parent who also has a mood disorder, usually Major Depressive Disorder. If one parent has Bipolar I Disorder, the child will have a 25% chance of developing a mood disorder (about half of these will have Bipolar I or II Disorder, while the other half will have Major Depressive Disorder). If both parents have Bipolar I Disorder, the child has a 50%-75% chance of developing a mood disorder. First-degree biological relatives of individuals with Bipolar I Disorder have elevated rates of Bipolar I Disorder (4%-24%), Bipolar II Disorder (1%-5%), and Major Depressive Disorder (4%-24%).
The finding that the concordance rate for monozygotic twins isn't 100% suggests that environmental or psychological factors likely play a role in causation. Certain environmental factors (e.g., antidepressant medication, antipsychotic medication, electroconvulsive therapy, stimulants) or certain illnesses (e.g., multiple sclerosis, brain tumor, hyperthyroidism) can trigger mania. Mania can be triggered by giving birth, sleep deprivation, and major stressful life events.