I haven't read the actual study, so I can't speak as to it's validity or not, but as someone who conducts research into the interplay between video games and psychology, I find it most concerning that Professor Anderson is advocating policy in order to prevent children from playing violent games. This is the job of the parent. If there is any measure that should be suggested, it should be one to promote better parenting, not censorship.
Again, please note that I have not read his study, but I am concerned with his display of the game "Manhunt," on his poster and his desk. Manhunt is already a controversially violent game, and was was given a Mature (18+) rating in the United States. As a fairly prolific gamer, I can qualitatively state that the majority of games, including most modern first person shooters, do not depict the level of violence and brutality shown in that game. The main point is that if games like Manhunt were used in Professor Anderson's studies, they are not representative of the majority of video games, and therefore his study may exhibit bias.
From the numerous (and often contradictory) studies I have read, I would not be surprised if violent videogames increased aggression, but given the general content of media these days I would tend to think this was more a symptom of greater problems within our society than an issue in-of itself.