Bipolar disorder is often considered a condition that reflects genetic influence. One twin study reveals just how genetic the disorder is.
If one identical twin develops bipolar disorder, the likelihood of the other twin developing the condition is 85 to 89 percent. The high probability of double trouble comes not from the environment the twins inhabited together while they were developing but from the genes they share, reports a team of British and Canadian investigators. Only 15 percent of instances of bipolar disorder can be attributed to factors specific to individuals or to their unique life experiences outside the family of origin.