http://psycnet.apa.org/journals/bul/136/2/151/
Here's a more recent meta-analysis that was done. I know you can only see the abstract, but the gist of it is that former accusations of publishing bias against previous studies regarding video games and aggressive behavior, lower helping behaviors, and desensitization don't hold statistical water. It also shows that these effects aren't culturally unique to the US, they also looked at cross cultural results with Asian countries and found that their findings were the same.
That being said, aggressive behaviors, reduced helping behaviors, and desensitization don't automatically equate to violent outcomes, but that is where interventions such as what Z is talking about come in. I know that my mother allowed me to watch all the slasher films I wanted, but she also made sure that I understood that it was fake, this actually led to an obsession with special effects on my part and seriously wanted to do that as a career at one point.
What follows is by own opinion based on my knowledge of interventions and other social psychology stuff:
I think that the effects of violence portrayals in all media can be mitigated by parents, social peers, and other important people within a child's social hemisphere nurturing that child/teenager and modeling pro-social behaviors. While many of us may have had those influences, think for a moment about kids who are entrenched in this stuff who don't have strong mentor figures, small to no social circles, and are largely ignored and bullied. That leads to a pretty potent cocktail, especially when you add in post pubescent hormones, and the poor decision making that comes from that developmental stage. So while I don't think that violent media leads to direct violent outcomes, it CAN be a contributor.
So, long story short, there's a simple solution.
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To me, books are far less desensitizing. Any visual is of your own making. Older movies, even as recently as the 90s are usually so hammy that it's hard to take it seriously. But the newer stuff, and video games where your character uses real weapons and people react in realistic ways, those are different to me. I for one was pretty shocked by the opening scene of Skyrim. My son saw it, despite my objections, and couldn't care less. Because it was nothing compared to the multiplayer CoD he plays at his friends' houses.
While I was writing my novel up there, you posted this. This has been looked at as well in research and desensitization from books is generally less than that in any sort of visual medium. It's a far different experience to read about graphic violence than it is to see it.[DOUBLEPOST=1411738185][/DOUBLEPOST]Also, I can tell from my own posts that I'm in full on research mode