Temperament traits of Novelty Seeking, Harm Avoidance, Reward Dependence, and Persistence, are well defined in terms of their neural circuitry, neurochemical modulators, and patterns of associative learning. When heritably excessive, each of these traits may become a mechanistically fundamental biogenetic trait vulnerability for personality disorder. The other main risk factor for personality disorder is environmental, notably abuse, neglect, and psychological trauma. The emerging concept of mechanism-based pharmacotherapy aims to activate the brain's homeostasis as the only available delivery system to re-calibrate complex neurophysiological participants in each of the temperament traits. In a positive feedback, a homeostasis-driven improvement of excessive temperament is expected to facilitate maturation of neocortical networks of cognition, most reliably in expert psychotherapy (Part I of this paper) and, ultimately, thereby improve top-down cortical control of subcortical affect reactivity. As an emerging concept informed by neuroscience and clinical research, mechanism-based pharmacotherapy has the potential to be superior to traditional symptom-based treatments. Such mechanism-based approach illustrates what the pharmacological treatment of Research Domain Criteria (RDoC) might look like.