We thought that aggressiveness as the reaction to peer rejection, so we have painstakingly attempted to eliminate peer rejection from the childhood experience. In its place is elaborately orchestrated peer interaction. We've created the play date phenomenon, while ladening older kids' schedules with after-school activities. We've segregated children by age--building separate playgrounds for the youngest children, and stratifying classes and teams. Unwittingly, we've put children into an echo chamber. Today's average middle schooler has a phenomenal 299 peer interactions a day. The average teen spends sixty hours a week surrounded by a peer group (and only sixteen hours a week surrounded by adults). This has created the perfect atmosphere for a different strain of aggression-virus to breed--one fed not by peer rejection, but fed by the need for peer status and social ranking. The more time peers spend together, the stronger this compulsion is to rank high, resulting in the hostility of one-upmanship. All those lessons about sharing and consideration can hardly compete. We wonder why it takes twenty years to teach a child how to conduct himself in polite society--overlooking the fact that we've essentially left our children to socialize themselves.
From Chapter Nine: Plays Well With Others in NurtureShock: New Thinking About Children by Po Bronson and Ashley Merryman