I believe people are born with this and are more likely to end up with BP. I also think we can be born with a higher chance to substance abuse. Studies did show that alcoholism changes your DNA therefore it can be passed to your children. I also think environmental issues play a role in bringing out MI. Just like how they say that people with schizophrenia became that way after a major trauma. I think we are born with a higher chance of becoming MI and a trauma in life can "wake it up" in us. That's my case at least.