The Social Face Hypothesis

Meeting the demands of a social world is an incredibly complex task. Since humans are able to navigate the social world so effortlessly, our ability to both interpret and signal complex social and emotional information is arguably shaped by evolutionary pressures. Dunbar (1992) tested this assumption in his Social Brain Hypothesis, observing that different primates' neocortical volume predicted their average social network size, suggesting that neocortical evolution was driven at least in part by social demands. Here we examined the Social Face Hypothesis, based on the assumption that the face co-evolved with the brain to signal more complex and nuanced emotional, mental, and behavioral states to others. Despite prior observations suggestive of this conclusion (e.g., Redican, 1982), it has not, to our knowledge, been empirically tested. To do this, we obtained updated metrics of primate facial musculature, facial hair bareness, average social network size, and average brain weight data for a large number of primate genera (N = 63). In this sample, we replicated Dunbar's original observation by finding that average brain weight predicted average social network size. Critically, we also found that perceived facial hair bareness predicted both group size and average brain weight. Finally, we found that all three variables acted as mediators, confirming a complex, interdependent relationship between primate social network size, primate brain weight, and primate facial hair bareness. These findings are consistent with the conclusion that the primate brain and face co-evolved in response to meeting the increased social demands of one's environment.

The online version contains supplementary material available at 10.1007/s42761-022-00116-7.