Department of Anthropology, Dartmouth College
Friday All day, Plaza Level
Psychologists have suggested that facial expressivity is signal of trustworthiness in humans. This is based on the observation that people are more likely to cooperate with individuals that readily produce facial displays. The “trustworthy face hypothesis” implies that facial expressivity evolved in correlation with cooperative behaviors. To test this hypothesis in catarrhines, I examined the co-evolution of facial motor control and social grooming, which is an example of reciprocal altruism.
Data were gathered from the literature (N = 10) and analyzed using phylogenetic GLS regression. The dependent variable was facial nucleus volume, which is a proxy for facial motor control. Medulla volume, social grooming time, and population group size were predictors. Social grooming time was a significant predictor of facial nucleus volume independent of medulla volume (b = 0.21, t = 2.65, p < 0.05). In contrast, the effect of group size on facial nucleus volume did not reach statistical significance after controlling for medulla volume (b = 0.14, t = 1.74, p = 0.07). When both social grooming time and group size were included as predictors in the same model, neither variable reached statistical significance, but the effect of grooming time was stronger than group size (social grooming: b = 0.22, t= 1.55, p =0.18; group size: b = -0.01, t = -0.08, p = 0.94).
The results of this study provide comparative support for the trustworthy face hypothesis. The importance of facial expressivity in cooperative interactions might be a productive area of future research in behavioral primatology.