There is a story, believed to be of Cherokee origin, in which a girl is troubled by a recurring dream in which two wolves fight viciously. Seeking an explanation, she goes to her grandfather, highly regarded for his wisdom, who explains that there are two forces within each of us, struggling for supremacy, one embodying peace and the other, war. At this, the girl is even more distressed, and asks her grandfather who wins. His answer: “The one you feed.”

http://www.nytimes.com/2013/09/29/opinion/sunday/are-we-hard-wired-for-war.html