The unilateralist's curse is the phenomenon whereby, when each of many altruistic agents has the power to bring about some state of affairs whose net value is negative but unknown to these agents, the probability that the state will be realized grows with the number of agents who decide to act based on their own private judgment.

Salient examples include decisions to leak information about weapons technologies, potential decisions by individual nations to use geoengineering to mitigate climate change, and the unilateral decision to introduce rabbits to Australia.

To avoid the unilateralist’s curse, members of a group might implement a group decision-making procedure, deliberate with others before taking action, or create a norm of deferring to the beliefs or actions of the other members of the group.