The Polyvagal Theory (gr. ‘polus’, “‘many’” + ‘vagal’, “‘Vagus Nerve'”) was proposed and developed by Dr. Stephen Porges, Director of the Brain-Body Center at the University of Illinois at Chicago. The theory specifies two functionally distinct branches of the vagus, or tenth cranial nerve. The branches of the vagal nerve serve different evolutionary stress responses in mammals: the more primitive branch elicits immobilization behaviors (e.g., feigning death), whereas the more evolved branch is linked to social communication and self-soothing behaviors. These functions follow a phylogenetic hierarchy, where the most primitive systems are activated only when the more evolved structures fail. These neural pathways regulate autonomic state and the expression of emotional and social behavior. Thus, according to this theory, physiological state dictates the range of behavior and psychological experience. Polyvagal theory has many implications for the study of stress, emotion, and social behavior, which has traditionally utilized more peripheral indices of arousal, such as heart rate and cortisol level. The measurement of vagal tone in humans has become a novel index of stress vulnerability and reactivity in many studies of populations with affective disorders, such as children with conduct problems and those suffering from borderline personality disorder.