Effects of procedural fairness and self-interest on cooperation vs. hardness in an urban planning conflict were examined in a questionnaire study. The self-interest perspective posits that people seek to maximize their outcomes and, therefore, cooperate only when they can gain advantages or avoid disadvantages by doing so, while fairness models of cooperation view cooperative actions as based mainly on justice motives. The data give strong support for the fairness hypothesis, showing that self-interest accounts only for hard strategies, whereas justice motives explain a considerable part of cooperation. The impact of the results on further research on social conflicts and their settlement, as well as the elaboration of models of fairness in the social psychology of conflicts are discussed.