I just read a very interesting article/interview regarding girls and being mean. There's an interview with Rosalind Wiseman, author of Queen Bees & Wannabes, which was the basis for the 2004 film "Mean Girls."
The advice seems fairly sound, much better than the "be nice" suggestion many parents might give. While we are taught to "kill them with kindness" (which is a rather harsh way of putting it, actually), I can't see the logic in being overly nice to someone who won't reciprocate. Why not just move on?
I don't readily believe, like some of the comments left at the end of the story, that we learn about being mean from our parents. It's hard for me to imagine a four-year-old learning to exclude people or consider someone bad from a parent. I know kids re-enact things they see their parents doing, or repeats words/phrases, but I don't picture myself (literally) pushing a person in the supermarket because they are a poopyhead.