If you saw someone being attacked in the street, how would you react? Social norms would dictate that such behavior is unacceptable in modern society and that someone should step in. But at what point did we, as a group, decide this? New research hints at the possibility that this behavior evolved early on, as chimpanzees might themselves show a form of “proto social norms.”

In an experiment involving two social groups of chimpanzees that live in two separate Swiss zoos, researchers from the University of Zurich played them a series of video clips. They found that the chimps spent much longer watching the film that showed chimpanzee infanticide than any other films that depicted other chimp behavior. The scientists suggest in a paper published in Human Nature that this could show that the apes were aware that what they were seeing was not considered acceptable.  

The videos played to the chimps showed a variety of different actions. Some of them were neutral, such as chimps walking or cracking nuts, but others were more aggressive. One of the scenes was of adult apes fighting, another of a baby colobus monkey being hunted and eaten, and a third of an infant chimp being killed by adults. The zoo animals were found to watch the third clip for up to four times longer than any of the other situations.

They were able to rule out the possibility that it was simply because the chimps had never witnessed such violence before, as both captive groups had experienced infanticide within their own community. Due to this, and the preferential attention they paid to the scenes of infants dying, they suggest that apes can distinguish between different types of violence, and that such horrific actions are vastly different to what the chimps might normally expect and hence why it got their attention.

But despite this, while the chimps did act emotionally to a certain extent, the researchers only saw limited evidence that they were willing to act on it. They say that this demonstrates how the chimps would act as bystanders to such violations of normal behavior, but it could also be because – like in humans – viewing such actions on a TV simply doesn’t induce the same emotional reaction as if the event is happening in front of them.  

“The results suggest that chimpanzees detect norm violations both within their group as well as in a group of unfamiliar individuals, but that they will only respond emotionally to such norm violations within their own group,” explains lead author Claudia Rudolf von Rohr.