The magic bullet theory-- or hypodermic needle theory--was much more common among media researchers in the early 20th century than it is now. This theory posits that media messages impact people in direct, measurable, and immediate ways-- as if a bullet hit the body, or as if the body was injected with a substance from a needle. Most researchers argue that these kinds of effects are rare, or involve events of little consequence. For example, when someone watches a pizza commercial and then orders the pizza seen on the TV, this is more or less a magic bullet effect. However, it's much less likely that someone will see a school shooting on TV and then immediately attack a school.

Desensitization

This theory posits that because people are exposed to so much violence in the media, violence no longer makes a strong emotional impact upon them. Most people would agree that by watching lots of violent movies, a viewer no longer gets upset while watching violent movies. However, the debate surrounding this concept is whether people will also be desensitized to real life violence. If a person leaves the movie theater after seeing a violent film, and then sees a real dead body on the street, will this person still experience desensitization?

Cultivation Theory

Cultivation theory focuses more on how people's attitudes are impacted by the media, rather than just behaviors. Although attitudes and behaviors are intricately related, cultivation theorists focus on how people think more than what people do. Much of this research involves comparing the attitudes of heavy media users, moderate media users, and light media users.

One finding of this research is that when people are exposed to heavy media violence, they seem to have an attitudinal misconception called mean world syndrome. This means that they overestimate how much violence actually occurs in their communities and the rest of the world. People who are exposed to less media violence have a more realistic sense of the amount of violence in the real world.

Sociological Approaches to Media Violence

A less common way of studying media violence is sociological. Sociological theories of media violence explore the ways that the media impact and reinforce dominant ideologies and values in a culture. For example, a researcher might look at correlations between media violence and attitudes about masculinity in a culture, or how media violence reinforces and reflects a nation's violent foreign policy. Sociological theories of media are not measurable-- rather, these are theoretical ways of looking at the media's relationship with culture.

http://medialiteracy.suite101.com/article.cfm/theories_of_violence_in_the_media