Any preference for my group's interests over yours must be justified by some unbiased, disinterested ethic. Which sounds simple but, given that we're dealing with Humans and not Vulcans, it's sometimes difficult for two parties to agree on basic principles, specially parties who are unable or unwilling to switch points of view. This is the power of ethical reasoning.
Michael Shermer, The Moral Arc: How Science and Reason Lead Humanity toward Truth, Justice, and Freedom
ethics