Recent comparative evidence suggests that anthropoid primates are the only vertebrates to exhibit a
quantitative relationship between relative brain size and social group size. In this paper, I attempt to
explain this pattern with regard to facial expressivity and social bonding. I hypothesize that facial
motor control increases as a secondary consequence of neocortical expansion owing to cortical
innervation of the facial motor nucleus. This is supported by new analyses demonstrating correlated
evolution between relative neocortex size and relative facial nucleus size. I also hypothesize that
increased facial motor control correlates with enhanced emotional expressivity, which provides
the opportunity for individuals to better gauge the trustworthiness of group members. This is supported
by previous evidence from human psychology, as well as new analyses demonstrating a
positive relationship between allogrooming and facial nucleus volume. I suggest new approaches
to the study of primate facial expressivity in light of these hypotheses.