Social Psychologists have identified a tendency for groups composed of like-minded individuals to make decisions that are more extreme, but biased in the same direction as decisions taken by individual members of the group. This tendency is a called the group-polarization phenomenon. One explanation for the phenomenon is the “persuasive argument theory.” Loosely, the persuasive argument theory asserts that individuals become more convinced on their view when they hear new arguments that support their position and that group deliberations bring out these arguments. I provide one formalization of this theory and investigate the extent to which the persuasive argument theory leads to polarization and conditions under which group decisions are not necessarily better than decisions made by group members. I argue that group polarization is not necessarily a sign of non-optimizing behavior and does not require persuasive arguments, but that either when there is a conflict of interest between decision makers or limits on the ability to communicate, novel arguments will receive disproportional weight in deliberations and may lead to biased group decision making.