Group decision support systems (GDSS) hold significant potential for improving decision making, but they have not been broadly adopted. One reason for this is that these platforms introduce representational work for users that is distinct from a more familiar deliberative interaction but they offer uncertain payoff. This article presents a study with a platform that addresses this problem by leveraging the argumentative structure of deliberative conversation to drive a decision support algorithm. The platform uses argument visualization to mediate the collaborators’ conversation. The study demonstrates that the platform addresses a known deficiency in human information pooling called the “common knowledge” phenomenon. Categories and Subject Descriptors H.5.3 [Information interfaces and presentation]: Group and Organization Interfaces---Computer-supported cooperative work, evaluation/methodology, synchronous interaction, web-based interaction; H.4.2 [Information systems applicati...