Read that link by Christina86. That's a good start.

It is definitely genetic. Not everyone develops it though. The trick is to figure out the vast array of initiation triggers. Abnormally high stress is a certain trigger.

Other than that? I dunno. I developed this at age 28. My sister developed it age 17. For me it was significant stress due to being a repeated victim of crime. For her, who knows. She was in high school and the most popular kid.

It's interesting to think about. What is the genetic purpose of Bipolar? Recent research indicated that other mental illnesses are some kind of evolutionary adaptation. If that's the case, I'd rather stay "primitive".