Children subjected over extended periods to repeated frustrations, rejection, and other aversive stimuli may eventually learn to strike back with aggression and perceive the world as a hostile place. By observing aggressive persons around them -- parents, peers, and television characters -- children acquire new modes of destructive and injurious behavior and learn that aggression is a legitimate and successful way of getting what one wants. Children who are permitted to express aggression, even occasionally, will develop habits of persistent and intense aggression that is difficult to modify. Several clinical psychologists have developed school-based intervention programs that recognize the aforementioned principles for the development of aggressive behavior. These treatment programs, which typically enlist the help of peer groups and teachers, are designed to teach aggressive children that their aggression will not be tolerated and that their needs can be met through nonviolent means.