Humans' left cheek portrait bias extends to chimpanzees: Depictions of chimps on Instagram

When posing for portraits, humans favour the left cheek. This preference is argued to stem from the left cheek's greater expressivity: as the left hemiface is predominantly controlled by the emotion-dominant right hemisphere, it expresses emotion more intensely than the right hemiface. Whether this left cheek bias extends to our closest primate relatives, chimpanzees, has yet to be determined. Given that humans and chimpanzees share the same oro-facial musculature and contralateral cortical innervation of the face, it appears probable that humans would also choose to depict chimps showing the more emotional left cheek. This paper thus examined portrait biases in images of chimpanzees. Two thousand photographs were sourced from Instagram's "Most Recent" feed using the #chimpanzee, and coded for pose orientation (left, right) and portrait type (head and torso, full body). As anticipated, there were significantly more left cheek (57.2%) than right cheek images (42.8%), with the bias observed across both head and torso and full body portraits. Thus humans choose to depict chimpanzees just as we depict ourselves: offering the left cheek. As such, these data confirm that the left cheek bias is observed in both human and non-human primates, consistent with an emotion-based account of the orientation preference.