http://www.mydaily.com/2011/02/25/mean-girls-explained-science-reveals-how-women-form-alliances/ It's not just a byproduct of our own imagination -- mean girls are REAL!
Obviously, we don't mean girls who are mean; we mean girls who form mobs with other girls and are mean together.
And, just like you always said after being forced to eat lunch on the toilet in middle school, they're doing it because they're the ones who are insecure.
According to a new study conducted by psychological scientist Joyce F. Benenson of Emmanuel College, ladies form alliances with each other to exclude somebody if they feel there's the threat of competition.
To achieve these results, men and women were placed in an imaginary game scenario wherein they could increase their chances of winning by forming an alliance with another player; more often than men, women chose this option over competing alone, even though it meant they would ultimately win fewer points.
To put it in science-y terms, borrowed from the study (which will be published in "Psychology Today"), "As their primary competitive strategy to combat any social threat, females may attempt to form an exclusionary alliance, whereas males may endeavor to unilaterally and directly dominate an opponent."