Neurobiological cause of intergroup conflict: 'Bonding hormone' drives aggression towards competing out-groups (altruism)

"The evolution of altruism in intergroup conflict
The research team at the University of Amsterdam, directed by Dr. Carsten de Dreu, wondered why oxytocin would promote altruistic behavior. Whereas classic economic theory has difficulty accounting for altruism, an evolutionary perspective suggests that altruism functions to strengthen one's own group, from which the individual benefits in the long run. Because aggression towards competing out-groups helps one's own group to become relatively stronger, aggression is an indirect form of altruistic, loyal behavior towards one's own group........Charles Darwin already observed that groups whose members are altruistic towards the own group have a greater likelihood to prosper, to survive, and spread...."