Mean Girls

Boys can be physically aggressive and violent, but mean girls can be aggressive in often subtler ways, and it starts early--at preschool or kindergarten. Social scientists describe this systematic teasing as "relational aggression" or "social cruelty." Relational aggression actually leads to physical symptoms, such as headaches or stomachaches, as well as often have more long-lasting effects than physical aggression.  

Unfortunately, these behaviours are often hidden in girl culture, so adults are often oblivious of what's happening.

One common aggressive behaviour displayed by mean girls is exclusion, one of the most hurtful actions for most people, and in particular, girls. These actions lead to problems to girls' psyches and self-esteem, as well as affect their learning and achievement. 

Girls are often stuck in a social system known as a yo-yo friendship; in other words, best friends one day, then worst enemies, and then best friends again--and on and on it goes. 

Source: Little Girls can be Mean, Michelle Anthony & Reyna Lindert, 2010.