This article on the NY Times website is one of the reasons why I was terrified when I found out I was having a girl. Take it from someone who moved 519 times growing up (or maybe 10), girls are vicious.  The article points out something I’d never seen anywhere else (but can attest to anecdotally):

“The girls who are the victims tend to be raised by parents who encourage them to be more age appropriate,” Ms. Rosenman said. “The mean girls are 8 but want to be 14, and their parents play along. They all want to be top dog.” And so the nastiness begins.

In my case, I was very young for school and then skipped a grade.  Add to that all those moves, and, well, I know a lot about mean girls.  I don’t remember them starting until fifth or sixth grade, though – the thought that P will have to start dealing with them in the next two or three years is chilling.