Great question mickeeteeze, here is a video that sheds a little bit of light on the subject.

Video from Howard Zinn, "On Human Nature and Aggression." From "You Can't Be Neutral On a Moving Train", 2004 [DVD].

Howard Zinn (born August 24, 1922) is an American historian, political scientist, social critic, activist and playwright, best known as author of the bestseller, A People's History of the United States.