Chimpanzees know social norms, especially tolerance to infant chimps. Source: Wikimedia Commons

Chimpanzees know social norms, especially tolerance to infant chimps. Source: Wikimedia Commons

According to a new June paper in Human Nature, the reactions of chimpanzees to videos showing violence against an infant chimp reveal that chimpanzees can recognize putative social norms. The chimpanzees did not, however, react in strongly negative ways – showing that chimpanzees may not intervene or protest if the violation does not occur within their community.

In order to help trace the evolutionary origin of human social norms, researchers at the University of Zurich studied chimpanzees to see if they could recognize and respond to violation of social norms in a regular manner. Previous observational studies have shown bystander chimpanzees enforce social norms using a variety of behaviors such as pacifying actions and protest vocalizations in reaction to violence within a group. This experiment is one of the first to systematically study the responses of chimpanzees to violations of social norms and thereby shed light on the evolution of uniquely human social norms.

According to the authors, the most suggestive evidence of social norms in chimpanzees is reactions to violence against infant chimps. The researchers studied two groups of chimpanzees living in Swiss zoological gardens by recording their reactions to multiple video clips. These clips showed either neutral behaviors, such as a chimpanzee cracking a nut, or violence towards an infant chimp. The violent clip was shown among other clips showing aggressive behavior between adult chimpanzees and the hunting of a colobus monkey. The chimpanzees were monitored with video cameras to observe the focus of their attention and their emotional reactions using looking times, which are commonly used to investigate nonhuman primates’ physical and social expectations. If an event is unexpected, chimpanzees tend to look for a longer period of time.

The experiment revealed that chimpanzees, as a whole, paid preferential attention to the infanticide scenes. In fact, the looking time was four times higher for the violence against infant chimps than for the other treatments and the control. The data indicates that chimpanzees recognize infanticide as a social norm.

Interestingly, the emotional reactions of the chimpanzees to violence against a chimpanzee infant were comparable to the neutral and treatment group measurements. This means that the chimpanzees did not act in a defensive or interventional way toward the infanticide of an infant chimp from another community. The authors suggest that, while chimpanzees recognize norm violations, they do not react emotionally if the incident does not occur within their group.

This exciting study shows the first evidence of chimpanzees adhering to social norms. According to the researchers, several building blocks of morality have been identified to some extent in chimpanzees. These blocks include consolation, inequity aversion, and spontaneous altruism. With further studies and experiments using more physiological measurements, more evidence of social norms in chimpanzees will come to light.

These results may have numerous implications, especially for social norms in humans. The authors drew multiple parallels between human and chimpanzee behavior, noting that this study could help illustrate the evolution of specific social norms in our own species itself.



1. Claudia Rudolf von Rohr, Carel P. van Schaik, Alexandra Kissling, Judith M. Burkart. Chimpanzees’ Bystander Reactions to Infanticide. Human Nature, 2015; DOI: 10.1007/s12110-015-9228-5