Reciprocity is considered one of the most potent weapons of social influence. Yet, little is known about when reciprocity appeals are more or less effective. A functional evolutionary approach suggests that reciprocity helps people survive in resource-scarce environments: When resources are limited, a person may not be able to obtain enough resources on their own, and reciprocal relationships can increase the odds of survival. If true, people concerned about resource scarcity may increasingly engage in reciprocal relationships and feel more compelled to reciprocate the favors done for them by others. In a series of experiments, I test this hypothesis and demonstrate that: (1) chronic concerns about resource scarcity (low socioeconomic status) predict increased reciprocity, (2) experimentally activating resource scarcity enhances the effectiveness of reciprocity appeals, (3) this effect is moderated by cues of persuasive intent, and (4) this relationship is mediated by increased gratitude.