What effects does watching violence have on people?

What effects does watching violence have on people? I think that would really depend of the person watching it. If the person is a mature adult and the violence is in a movie for example I feel is would just be for entertainment. When I watch a movie, like the one I saw today, (The Taking of Pelham 123), this was a very violent movie with a lot of shooting and killing, but I did not leave the movie feeling the need to be violent. I have six grandchildren from ages 4 months to nine years and I use their age as a gauge when I decide what movies are OK for them. Some of the movies that are for children have violence especially the super-hero movies. I have taken some of my grandchildren to see these movies and they know they are just movies. I have not seen any signs that the violence has affected them in any way. If we are talking about real life violence like when a young child is a witness to an abusive parent I feel differently. I do believe that young children are affected in many ways by this type of violence.