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.

What's more, the genetic loading for bipolar disorder is restricted to liability to mania.

Interestingly, the study also demonstrates that unipolar depression is not simply a less severe form of bipolar disorder. Only 10 percent of twins with unipolar major depression had a co-twin with bipolar depression.

Nevertheless, there is some genetic overlap; susceptibility to bipolar disorder seems to make sufferers susceptible to unipolar depression as well. Fifty percent of the co-twins with bipolar disorder also had unipolar disorder.

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