A recent study out of Université Pierre-Mendès-France, conducted with researchers from Ohio State University and the University of Hohenheim suggests that there may be a cumulative effect in aggression and the expectation of hostility among players of violent video games.
The researchers assigned 70 undergraduate students to play either violent video games (Condemned 2, Call of Duty 4, or The Club) or non-violent video games (S3K Superbike, Pure, or Dirt 2) for twenty minutes on three consecutive days. After each session, participants were asked to read the beginning of a story and then come up with twenty possible actions for the main character to take. Researchers then coded participants' responses, counting how many aggressive or violent actions were included. Subsequently, participants were told they would engage in a competitive task with an unseen other participant (there was no other participant); participants were told they could send the "other player" a noise blast through their headphones, and that they could choose how long and loud it would be.
The researchers found that those who played the violent video games gave more aggresive/violent actions for the story's main character, and gave the fake other participant a louder, longer blast of noise. Brad Bushman, one of the lead researchers, holds that:
People who have a steady diet of playing these violent games may come to see the world as a hostile and violent place...These results suggest there could be a cumulative effect.
Playing video games could be compared to smoking cigarettes. A single cigarette won't cause lung cancer, but smoking over weeks or months or years greatly increases the risk. In the same way, repeated exposure to violent video games may have a cumulative effect on aggression.
Unfortunately, none of the reports linked above mentions the size of the effect, only that it is statistically significant. But a statistically-significant increase can be anywhere from a fraction of a percent, to a thousand-fold, or more. Data from the study's publication in the Journal of Experimental Social Science suggest the violent-video-game group was about twice as hostile and twice as aggressive on the study's measures as the non-violent-game group after three days. What happens after three days, however, is anybody's guess.
Interestingly, the results of this study contradict earlier research that suggests the degree of competitiveness a game, rather than the degree of violence, is responsible for the increase in aggression.