In step-level public good dilemmas the equality rule serves as an important distribution rule to tacitly coordinate group members' decisions. In two studies, we examined the motives that may underlie the use of the equality rule. More specifically, we examined whether people use the equality rule out of fairness concerns or out of efficiency concerns. For this purpose, we assessed people's emotional reactions toward a violator of the equality rule when the group succeeded vs. failed, as a function of social value orientation. The results of both experiments showed that proselfs' emotional reactions towards a violator were a function of the success or the failure of the group, whereas prosocials' emotional reactions did not vary as a function of the outcome feedback. These results suggest that prosocials prefer the equality rule out of fairness concerns whereas for proselfs efficiency concerns dominate. Copyright © 2005 John Wiley & Sons, Ltd.