There is an emerging body of research that suggests that aggressive behavior is socially rewarded by other children-- in the article you posted, in kids as young as three-- in this research, by 4th to 6th graders:http://www.selfhelpmagazine.com/arti...gressboys.html
I think it's really clear by watching toddlers that they understand that aggression (whether physical aggression or the "relational" (mean words) aggression) has a big impact on others-- your peers and the adults around you. Even when children are parented without physical punishment, they can quickly figure out the rewards of hitting, biting, etc.
We should not be surprised by the use of aggression in children, even young ones. Adult men who batter do not grow up and start battering for the first time in their lives-- rather, they've practiced it on their siblings, playmates, and classmates.
In my son's former preschool/daycare, there was lots of relational aggression by the 2-3 year old crowd (usually girls). Both boys and girls were really into social power, and they found out that one of the ways to acquire it was by demeaning and excluding other kids. It was icky.
As long as our culture rewards aggression, we will continue to see it from everybody.