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.
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We thank Elizabeth Lonsdorf and Ian Gilby for generously sharing their film footage with us, and Igor Drieghe for his help with the video standardization. We also thank the Walter Zoo in Gossau and Zoo Basel, and especially animal keepers Angela Widmer and Stefan Lopez for their cooperation during the experiments, and Corinne Sober and Angela Ziltener for assistance with the experiments and Sereina Graber and Alexandra Kropac for assistance with the data analysis. We also thank Sonja Koski and two anonymous referees for fruitful discussions and comments on an earlier version of this manuscript. This work was funded by the University Priority Research Program in Ethics at the University of Zurich and the A. H. Schultz-Foundation of the Anthropological Institute and Museum at the University of Zurich.
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von Rohr, C.R., van Schaik, C.P., Kissling, A. et al. Chimpanzees’ Bystander Reactions to Infanticide. Hum Nat 26, 143–160 (2015). https://doi.org/10.1007/s12110-015-9228-5
- Chimpanzees (Pan troglodytes)
- Expectancy violation
- Social expectations
- Moral behavior
- Evolutionary preconditions of morality
- Social norms