Researchers say that heritability — how much of bipolar disorder is due to genetics — accounts for between 60 and 70 percent of the risk of getting the illness. Genetic studies reveal the following:
The identical twin of a person with bipolar I disorder has about a 50 percent risk of developing the disorder. Because identical twins share all their genes, this finding means genetics accounts for some, but not all, of the chance of developing bipolar disorder. (If the genetics were fully responsible and one identical twin had bipolar I, then the other twin would have a 100 percent chance of developing the disorder.)
If someone has an immediate family member with bipolar I disorder, the person has about a 10 percent chance of developing the disorder, which is much higher than the 1 to 2 percent chance of having the disorder if for an individual with no family history of bipolar.
The familial risks of bipolar II and cyclothymia — so called bipolar spectrum disorders — are less clear in the research. First-degree family members (parents, siblings, or offspring) of people with bipolar disorder are more likely to have other disorders as well, such as unipolar depression, schizophrenia, autism disorder, anxiety disorders, substance abuse, attention deficit hyperactivity disorder (ADHD), and personality disorders.