Borderline personality disorder (BPD) is a personality disorder described as a prolonged disturbance of personality function in a person (generally over the age of eighteen years, although it is also found in adolescents), characterized by depth and variability of moods. The disorder typically involves unusual levels of instability in mood; black and white thinking, or splitting; the disorder often manifests itself inidealization and devaluation episodes, as well as chaotic and unstable interpersonal relationships, self-image, identity, and behavior; as well as a disturbance in the individual's sense of self. In extreme cases, this disturbance in the sense of self can lead to periods of dissociation.

BPD splitting includes a switch between idealizing and demonizing others. This, combined with mood disturbances, can undermine relationships with family, friends, and co-workers. BPD disturbances also include self-harm. Without treatment, symptoms may worsen, leading (in extreme cases) to suicide attempts.