Have twin studies shown that bipolar disorder is hereditary?

Yes. The disorder is highly heritable. Estimates of the genetic component to the heritability range from 60 to 85 percent.
Bipolar disorder. It is highly genetic illness. There is about 70 % chance of child to have bipolar disorder if both parents have been diagnosed with it as well.

Related Questions

My previous child (now 16 yrs) has bipolar disorder. What preventive measures/drugs can I take to avoid it for my twins (currently 29th week)?

Bipolar Disorder. There are no drugs which prevent bipolar disorder. It is largely genetic. However, consistent parenting skills, nonuse of corporal punishment, consistent and age appropriate discipline, good sleep hygeine and nutrition are all important for mental health in general. Hope this helps!