Decades have passed since the inception of the fi eld of Human-Computer Interaction. The emergence of the Internet has shifted researchers' attention from understanding how individuals interact with computers to understanding how individuals interact with one another using computer technologies. A wide range of systems designed for multiple users have been labeled as groupware, collaborative computing, multiuser applications, and more recently social computing technologies. Designing these

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... of system is more challenging than designing singleuser ones because other people and their behaviors are integral elements of the system as experienced by users (Grudin, 1994 ) . The system itself, therefore, is nondeterministic and evolutionary because the experience of some users is partly the result of decisions that earlier users have made. Because the behavior of a multiuser system is not stable until a critical mass of users has developed a routine way of using it, it is diffi cult to predict how groups of users will respond to a particular design before the stable state is reached. As a result, interactive design and evaluation cycle, perhaps the most successful HCI technique for system design, is insuffi cient for the design of multiuser applications. Consider the design of an online health support group like breastcancer.org and the decision about whether to employ moderators who ensure group members spend their time discussing cancer-related topics, channel off-topic content to sub-forums, or prevent users from posting advertisements. A member's decision to participate in