Many hormones influence canine aggression, according to a new study by researchers at the University of Arizona titled, “Endogenous Oxytocin, Vasopressin, and Aggression in Domestic Dogs”. This is no surprise given that the hormones testosterone and serotonin have a huge influence on aggressive behavior, but this study provides evidence that high vasopressin levels are associated with aggression, and that high levels of oxytocin are associated with the absence of aggression in dogs. Previous work has shown that oxytocin levels in dogs are elevated by positive interactions with people. (In humans, oxytocin is important in both childbirth and in breastfeeding, and is also known to facilitate social bonding. Vasopressin is also influential in people, with previous research indicating that people with long standing aggression problems have high levels of this hormone.)
via Whisker Therapy