Research has clearly shown that violent video games can increase aggression. It is less clear why they do. This study investigates the mediating effect of the hostile expectation bias (i.e., tendency to perceive hostile intent on the part of others) on the link between violent video game exposure and aggression. French college students (N= 136) played either a violent or nonviolent game for 20. minutes. Afterwards, they read ambiguous story stems about potential interpersonal conflicts, and listed what they thought the main characters would do or say, think, and feel as the story continued. Aggression was measured using a competitive computer game in which the winner could blast the loser with loud noise through headphones. As hypothesized, video game violence increased the hostile expectation bias, which, in turn, increased aggression. Effects were larger for men than women. Thus one reason why violent games increase aggression is because they increase hostile expectations. © 2012.