Facial mobility, or the variety of facial movements a species can produce, is likely influenced by selection for facial expression in diurnal anthropoids. The purpose of this study is to examine socioecological correlates of facial mobility independent of body size, focusing on social group size and arboreality as possible evolutionary agents. Group size was chosen because facial expressions are important for group cohesion, while arboreality may limit the utility of facial expressions. Data for 12 nonhuman anthropoid species were taken from previous studies and analyzed using a phylogenetic generalized least-squares approach. Regression results indicate that group size is a good predictor of facial mobility independent of body size. No statistical support was found for the hypothesis that arboreality constrains the evolution of facial mobility. The correlation between facial mobility and group size may be a consequence of selection for more effective facial expression to help manage conflicts and facilitate bonding in larger groups. These findings support the hypothesis that the ultimate function of facial expression is related to group cohesion. Am J Phys Anthropol 2009. © 2009 Wiley-Liss, Inc.