A researchers has explained that since girls are often taught not to be aggressive, their hostility and anger are often expressed through their meanness.
"Girls are not brought up to be assertive. They're raised to be nice and pretty and have lots of friends," said Nicole Landry, a research coordinator with the Department of Community Health and Epidemiology in the Faculty of Medicine.
"But they themselves recognize meanness as an integral part, even a normal part, of their growing up," she added.
They tend to bully by gossiping, backstabbing, and excluding others from activities.
"Kids are like little adults, but they don't have the things that we have to give them status and power: a good job, a nice house, wealth. They use meanness as a way of negotiating their place in the hierarchy," she said.
"It's what girls do to get by. They need to dress the part and look the part and gather their army of friends around them. Their capital is their friends, their hair, their name-brand clothing-that's power for them," she added.
During the study, Landry looked at 24 tween girls, ages eight to 11.
She initiated discussion by showing movie clips and pictures and asking questions.
Each of the girls was also asked to record her thoughts and feelings in a "reflection journal".
According to the girls, popularity is affected by class and race; popularity, which is equated with power, is awarded to rich, white girls who can afford the coveted labels but also to white girls from less-well-off families as long as they are pretty.
But one thing the participants stressed about popular girls was that they were always mean; that's how they maintained their place at the top.
At the same time, these popular girls were inundated with rules, about how they must look, behave and who they could associate with.
According to the girls, some of rules for popular girls include: Always sass everyone;" "Get boys to like you;" and "Whenever you have a chance to make fun of someone else, do it."
The findings have been published in the book, The Mean Girl Motive: Negotiating Power and Femininity.