Social norms-generalized expectations about how others should behave in a given context-implicitly guide human social life. However, their existence becomes explicit when they are violated because norm violations provoke negative reactions, even from personally uninvolved bystanders. To explore the evolutionary origin of human social norms, we presented chimpanzees with videos depicting a putative norm violation: unfamiliar conspecifics engaging in infanticidal attacks on an infant chimpanzee. The chimpanzees looked far longer at infanticide scenes than at control videos showing nut cracking, hunting a colobus monkey, or displays and aggression among adult males. Furthermore, several alternative explanations for this looking pattern could be ruled out. However, infanticide scenes did not generally elicit higher arousal. We propose that chimpanzees as uninvolved bystanders may detect norm violations but may restrict emotional reactions to such situations to in-group contexts. We discuss the implications for the evolution of human morality.