As far a symptoms are concerned, I think an always unstable emotional state is the most characteristic. To me, borderline hints at being near a border, near a place where you can quickly go from one state to another. As to causes, I think probably being brought up in an atmosphere that was itself unstable, always on the edge of crisis. In such an atmosphere there is no chance to develop what they call "self-regulation" or self-soothing. In a family where every stumble is seen as a moral outrage, nothing is ever quiet; life is always charged with a feeling of danger. One always has to be prepared for an instantaneous response, to be on a hair trigger basis. Very much on a "war" footing.