Bi-polar disorder in children

For the general population, a conservative estimate of an individual’s risk of having full-blown bipolar disorder is one percent. Disorders in the bipolar spectrum may affect 4 to 6 percent. When one parent has bipolar disorder, the risk to each child is 15 to 30 percent. When both parents have bipolar disorder, the risk increases to 50 to 75 percent. The risk in siblings and fraternal twins is 15 to 25 percent. The risk in identical twins is approximately 70 percent.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2695748/