Abstract. The purpose of this paper is to investigate how argumentation processes among a group of agents may affect the outcome of group judgments. In particular we will focus on prediction markets (also called information markets) and we will investigate how the existence of social networks (that allow agents to argue with one another to improve their individual predictions) effect on group judgments. Social networks allow agents to exchange information about the group judgment by arguing about the most likely choice based on their individual experience. We develop an argumentation-based deliberation process by which the agents acquire new and relevant information. Finally, we experimentally assess how different social network connectivity affect group judgment.