My understanding is genetics is by far the most common cause, which can be without any know family history. Stress and trauma can be responsible for either bringing our existing genetics, or in extreme cases actually causing damage that is the actual source. This trauma can be physical or emotional, and the physical can be injury, or abuse, especially sexual abuse. Chemical abuse is another. Some do not recognize these cases as "true" BP, especially drug or alcohol, others do, but it really matters not if the symptoms are the same. But then our system is often very poor at correct DX in any case. I believe for many doctors the differentiation between, PTSD and such, and BP is as much the believed permanency of the condition and required medicine treatment. PTSD can often be treated with therapy and short term medication, while BP normally cannot, it require long term treatment.