Human assessment of chimpanzee facial asymmetry.

In this study, facial asymmetry in chimpanzees was assessed using a technique that has traditionally been implemented in human studies. Image composites made of each half of chimpanzees' facial expressions were presented to humans with and without chimpanzee experience. The group of subjects with chimpanzee experience considered composites made of the left side of the chimpanzee faces as the most emotionally intense for the emotional categories of play, silent bared-teeth, scream face, and a neutral category. On the other hand, left-left composites were not consistently judged by subjects with and without chimpanzee experience as the most similar to the whole original face, which might be explained as the result of an attentional bias in the human observers towards the right side of the chimpanzee expressions. Furthermore, responses given by subjects with and without chimpanzee experience were highly correlated, which indicates that the two groups of humans perceived the chimpanzee facial expressions in a similar fashion. The finding of left-sided asymmetries in these chimpanzees' facial expressions suggests a right hemisphere asymmetry in the production of emotions in this species and it is consistent with results reported in human and other nonhuman primates.