A new study observing the reactions of chimpanzees to infanticide videos may explain humans' sense of morality and the evolution of social norms.

The goal of the study was to detect possible prototypes of social norms by determining the reactions of uninvolved chimpanzee bystanders to a presumptive social norm and evaluating for the potential availability of social anticipation.

The study, led by Claudia Rudolf von Rohr of the University of Zurich in Switzerland, involved a total of 17 chimpanzee subjects in two Swiss zoological facilities in Gossau and Basel. The experiment consists of two stages. The first stage is called the habituation phase, which involved a film showing of unfamiliar conspecifics performing neutral activities such as walking and cracking nuts. The study subjects were made to watch the said videos once a day on three consecutive days for two consecutive weeks.

The second part of the test, which is called the experimental phase, involved the presentation of videos that feature unfamiliar chimpanzees under the following conditions: infanticide (violence against infants); hunt (including small monkey killings) and aggression (general socially aggressive behaviors). Each clip was shown in a counterbalanced order for three consecutive days in a week, lasting for a total of six weeks.

The results of the study, published in the journal Human Nature, reveal that the pattern of reaction during the first session of showing were fairly the same throughout the first week; however, compared to the rest of the six sessions, the reaction was much stronger. Viewing the infanticide video elicited a response that is four times stronger than the other videos.

The researchers also found that the chimpanzees were able to distinguish between severe aggression that comes along the infanticide video and other forms of aggression and neutral behavior. The chimpanzees notably looked at the infanticide video longer than the any other clips; this suggests that the amount of violence shown to the infants in the video did not match the social norm that the chimpanzees usually associate with infant treatment.

The researchers said that although the chimpanzees showed more reaction to the infanticide videos, there was still a lack of evidence to establish the higher amounts of arousal. The three main lacking points that the researchers found in their study include the lack of the realistic feature of television, unrefined set of indicators used to measure reactions (researchers said using physiological parameters such as heart rate and skin temperature may be more accurate) and lastly, the role of the chimpanzees as mere bystanders, whose group is not directly affected by the video.

This study is the first to show that chimpanzees are similar to humans in terms of sensitivity to acceptable behaviors, specifically those involving infants, and motivation to react only if they are directly affected.

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