Resumen: In a lab-in-the-field experiment, we study how the pro-social behavior of inhabitants of an impoverished neighborhood in Namibia is influenced after being informed about the pro-social or egoistic behavior of either a rich or a poor comparison group. We find that the poor behave significantly less prosocial when they learn about the egoistic behavior of the rich. Yet, neither the rich’s pro-social behavior nor information on how other poor individuals behaved affects the poor’s behavior. This contagion effect is not simply driven by imitating the behavior of the comparison group, or by social identity concerns or peer pressure. Instead, our data suggest that the poor’s drop in pro-social behavior is caused by the violation of a social justice norm: The poor expect the rich to be pro-social and they are surprised if they act differently. Learning about the rich’s egoism even makes participants’ beliefs about what should be done (injunctive norms) significantly more egoistic. Hence, the self-serving behavior of the rich causes a double damage: Society not only suffers from their low pro-sociality but also from a spread of egoism among other members of society.