Spontaneous facial expressivity predicts real-world social network size and richness

Humans have highly expressive faces, yet from an evolutionary perspective, the adaptive benefit of such expressivity remains unclear. One possibility is that facial expressivity confers social benefits. Here, we provide empirical support for this idea by demonstrating associations between individual-level facial expressivity and social network metrics. We recorded spontaneous social interaction across five studies (N = 1039) and extracted quantitative expressivity indices using the automated facial action coding system. We analyzed expressivity in relation to real-world social networks. Increased expressivity predicted larger, more diverse, and more embedded social networks in women, but not men. However, expressivity predicted more one-to-one social relationships across genders. Although correlational, these findings could suggest that facial expressivity functions to foster more intimate social connections. This provides support for the hypothesis that the need for close interpersonal bonds has driven the evolution of high levels of facial expressivity in humans.