Experts don't have a complete understanding of the exact causes of bipolar disorder, but the disorder appears to run in families, according to WebMD. Bipolar disorder is more likely to occur among the first-degree relatives of individuals with the condition. Bipolar II disorder is the most common form to affect families.
Studies of identical twins with bipolar disorder suggest that an identical twin has a 40 to 70 percent chance of developing bipolar disorder if the other twin has the mental illness, informs WebMD. A sibling of an individual with bipolar disorder has less chance of developing the condition than the affected individual's identical twin.
Individuals also have an increased risk of developing bipolar disorder if at least one parent has the illness, notes Health. Children with at least one bipolar parent have a 4 percent to 15 percent chance of developing bipolar disorder. Children are 3.5 times more likely to develop bipolar disorder if both parents are affected with the condition.
Although bipolar disorder appears to run in families, having a first-degree relative with the disorder does not mean a person definitively develops the disorder, adds Health. Additional factors, including a person’s stress levels and medical health, can influence his overall risk of developing bipolar disorder.