Children learn early on in their development that there are values to being perceived as aggressive. When they are young, the "roughhousing" one might see on a playground or in a backyard allows children to develop affiliations with others, helps them select their friends, establishes their place in the natural dominance hierarchy and helps them develop what are often very useful fighting skills. This does not mean that children who engage in this behavior, whether it is physically or verbally aggressive, intend to carry out any "threats" they may make in this process of identity development. It is only when this "rough and tumble" behavior persists in pursuit of outright domination that it becomes a problem.
As children mature, their behavior becomes more intentional. Exposure to social diversity, coupled with the overwhelming need to be respected and to feel a sense of belonging in a peer group increases the intensity of rough or aggressive interaction. The competition for status that young people engage in can become increasingly vicious. The insistence on being dominant turns into bullying, which can, in turn, develop into antisocial and/or aggressive behavior. If a youth does not find an alternative means of establishing his or her social position (such as success in school, music, sports, etc.), it is more likely that he or she will continue to use the means that have worked up to this point--aggressive behavior.

Dr. Arnold Goldstein, a leading practitioner, educator and researcher in the field of aggression management/replacement training and pro-social skills development, published some excellent books that explore the development of aggression in young people and ways in which aggressive behaviors can be managed and replaced with more functional behaviors. When looking at where aggressive behaviors stem from, Dr. Goldstein and his coauthors posit that aggression is primarily learned behavior; learned by observation, imitation, direct experience and rehearsal. Coercive parenting characterized by irritability and inconsistency is blamed for producing "temperamental" children who throw tantrums to get what they want. You'll usually see this at ages 2-3, also known as the "terrible twos". Children learn to be aggressive by watching what happens in the home and discovering that aggressive behavior can get them what they want. Once they get a couple of years older and are on the playground with their friends, they revert to what they know and use aggressive behaviors to get the toys they want and dominate their peers. Now these children begin to be labeled as "oppositional" or "problem children". Other kids who have not learned these aggressive behaviors shy away from befriending those who are bullying them, and the aggressive children find one another. They form a peer group that encourages this behavior and normalizes aggressive behavior as an acceptable way to gain attention. As the years go by, these aggressive behaviors become more intense, more frequent, and lead to children having "conduct disorders" and becoming "juvenile delinquents". (This is just a thumbnail sketch of the process that Dr. Goldstein describes in his book, Aggression Replacement Training.)
In addition to the home, schools and the media also have significant effects on aggressive behavior. Children may have direct learning experiences, during which they themselves are involved in aggressive behavior (as a victim or perpetrator), or vicarious learning experiences, where they watch aggressive interactions of others. The mass media has had a truly profound effect on children. Dr. Goldstein highlights three different consequences that children face after significant exposure to violent media such as television, movies, magazines, comic books and video games:
aggression effect: when kids copycat what they see in the media and carry out self-directed violence
victim effect: a sense of mistrust and anxiety about ones own safety and security
bystander effect: a callousness and loss of sensitivity toward violence
To learn more about the effects that the media has had on aggressive behavior in children, check out:
For more information on aggression replacement and pro-social skills training, check out these and other books by Dr. Arnold P. Goldstein. You can find them all at www.amazon.com.

Skillstreaming the Elementary School Child: New Strategies and Perspectives for Teaching Prosocial Skills by Ellen McGinnis, Arnold P. Goldstein. December 2000.

Skillstreaming the Adolescent: New Strategies and Perspectives for Teaching Prosocial Skills by Arnold P. Goldstein, Ellen McGinnis. August 1997.

Aggression Replacement Training: A Comprehensive Intervention for Aggressive Youth by Arnold P. Goldstein, et al. August 1998.

The Prosocial Gang: Implementing Aggression Replacement Training by Arnold P. Goldstein, et al. August 1994.