Dartmouth College anthropology professor Seth Dobson recently found a correlation between group interaction and the use of facial expressions in nonhuman primates. The findings are scheduled to be published this month in the American Journal of Physical Anthropology.
The research goal was to draw conclusions about the evolutionary role of facial expressions in social interaction, specifically focusing on social group size and arboreality (state of living in trees). The data came from previous studies of 12 nonhuman anthropoid species. According to the article, Dobson’s research found that “the ultimate function of facial expression is related to group cohesion.”
As Dobson explained in his paper, facial expressions are useful in communicating a range of information, from dominant/submissive to affinitive signals, thus serving as an important tool in many aspects of social interaction. Group cohesion is strengthened by increased communication abilities, and the evolution of complex facial expressions in anthropoids, including humans, is important in the context of social organization in large groups. For example, in different primate species, intragroup conflict is limited with the use of reconciliatory facial expressions, just as physical aggression is inhibited by the use of appeasing facial expressions.
Building an evolutionary model for the development of anthropoid facial expressions begins with understanding how facial expressions function in primate groups today. Drawing on a series of observational studies of nonhuman primates, Dobson developed a method of systematically interpreting these observations.
Dobson used a statistical analysis of these observational studies based on four variables: facial mobility, body mass, group size, and arboreality. He measured facial mobility according to the Facial Action Coding System (FACS) on a scale that quantifies observable facial expressions into in action units (AUs).
His findings revealed that larger-bodied, terrestrial species living in large social groups are correlated with the use of a wider and more sophisticated system of facial expressions. However, through preliminary analysis Dobson illustrated that facial mobility can be controlled for variation in body size. While his results showed that “no statistical support was found for the hypothesis that arboreality constrains the evolution of facial mobility,” his model demonstrated that “group size is a good predictor of facial mobility independent of body size.”
Dobson’s research provides insight into the driving factors that led to the extensive use of facial communication in primates. The high reliance on facial expression among primates, including humans, may be a key factor that supports large group organization within these species.