We have a great article, under glass on our kitchen table, written by Daniel Gilbert back in July 2006.
http://www.randomhouse.com/kvpa/gilbert/blog/ (the article starts about half way down the page)
The author talks about fighting with his brother in the back seat of the car. His mother would intervene leading to the inevitable, “But he hit me first,” one of us would say, to which the other would inevitably add, “But he hit me harder.”
"In virtually every human society, “He hit me first” provides an acceptable rationale for doing that which is otherwise forbidden." "That’s why participants in every one of the globe’s intractable conflicts — from Ireland to the Middle East — offer the even-numberedness ("my punch is the second punch, the retaliation") of their punches as grounds for exculpation."
"The problem with the principle of even-numberedness is that people count differently. Every action has a cause and a consequence: something that led to it and something that followed from it. But research shows that while people think of their own actions as the consequences of what came before, they think of other people’s actions as the causes of what came later.
Studies have shown that people remember what caused them to do or say something and they remember the consequences on them of what their opponent did or said. But they tend not to be aware of or remember what caused their opponent to do or say something to them or the consequences of their own action from the perspective of their opponent.
"What seems like a grossly self-serving pattern of remembering is actually the product of two innocent facts. First, because our senses point outward, we can observe other people’s actions but not our own. Second, because mental life is a private affair, we can observe our own thoughts but not the thoughts of others. Together, these facts suggest that our reasons for punching will always be more salient to us than the punches themselves — but that the opposite will be true of other people’s reasons and other people’s punches."