A 2008 experimental study involving children investigated the relationship between playing vs. watching violent video games on subsequent aggressive behavior. The study revealed that playing violent video games led to more aggression in boys. The same was not true of girls. What we as adults need to determine is what happens when these gamers continue gaming as adults, for the majority of gamers are not children, but adult males.
Polman H, de Castro BO, van Aken MA. Experimental study of the differential effects of playing versus watching violent video games on children’s aggressive behavior. Aggress Behav. 2008 May-Jun;34(3):256-64. PubMed PMID: 18161877. http://www.ncbi.nlm.nih.gov/pubmed/18161877