Unanimity rule is an important benchmark for evaluating outcomes of decisions in the social sciences. However, organizational researchers tend to ignore unanimous decision making, for example, because unanimity may be difficult to realize in large groups and may suffer from individual participants blocking decisions. This paper reconsiders unanimity rule in view of the development of circular systems for organizing decision making. It focuses on developing a theory of decision making under unanimity rule. The author uses a system dynamics model to explore the conditions under which unanimity rule supports the organization's ability to make decisions. Simulation experiments suggest that the dynamics and outcomes of unanimous decision making under and exceeding a critical threshold level of decision pressure are fundamentally different. Under this critical threshold, the decision-making system is capable of recovering from severe shocks to the system. If decision pressure is close to its threshold, a relatively small change can cause the decision process to collapse. In this respect, large groups operating under unanimity rule are less sustainable because they are more likely to exceed their critical threshold than small groups. Decomposing the decision-making system in small units, embedded in a hierarchical structure, therefore appears to be a necessary condition for sustainable performance in large organizations applying unanimity rule.