Psychologists Markus Appel and Susanne Jodbauer reviewed a large body of literature examining the link between violence in the media and how this shapes aggression. They observed two trends:
“(a) Media violence increases the likelihood of aggressive thoughts, feelings and behaviour among the audience, short-term and long-term. The magnitude of the effect depends on person, product and situation characteristics. (b) Media violence is not the only, and likely not the most important, factor contributing to aggressive thoughts, feelings and behaviour.”
Family socialisation, culture, gender and other factors play a stronger determining role on how people play out violent behaviour. This is why education is paramount in managing and preventing aggression.
Read the research: “The Effects of Media Violence,” in the Encyclopaedia of Peace Psychology.