Mostly nature, I think, although nurture must have played into it. Both sides of my family have a marked history of mental illness. OCD, bipolar disorder, depression, suicides, institutionalizations, etc. My dad has BP2 and depression, like I, and his illness was always a source of tension and worry in our household. Obviously, the genetic presence of mental illness in a family can create an atmosphere that "nurtures" (i.e. aggravates) the illness(es).