My fathers side is clear but my mothers side...she was bipolar or depressed (commited suicide), my moms sister was bipolar (also commited sui), my other moms sister is bipolar I, and my moms 3rd sister is also bipolar w/ other issues. My moms daughter from another father is also bipolar I. And that's just the family history I know!
There's a very strong genetic link on my mothers side. How much that plays a factor? I don't know but at least some I would think.