On prototypical facial expressions vs variation in facial behavior: what have we learned on the “visibility” of emotions from measuring facial actions in humans and apes

It has long been recognized that behavior evolves as do other traits and that it may have great impact on evolution. It tends to be conservative when survival and fast responding are at stake, and because of that, similar patterns can be found across populations or species, typical in their form and intensity, and often also typical in context and consequence. Such fixed stereotypic patterns that evolved to communicate are known as displays, and their phylogenies can virtually be traced. In this chapter, we contrast and discuss two coexisting trends in the study of the meaning and origins of human facial expression: one, with a tradition of exploring cross-cultural commonalities in the recognition of facial expression, that may indicate species-specific displays of emotion (prototypical facial expressions) and another that builds upon the growing evidence that such expressive prototypes are outnumbered by a diversity of facial compositions that, even in emotional situations, vary in relation to culture, context, group, maturation, and individual factors. We present behavioral studies that look at links between basic emotion and facial actions in both human and non-human primates and discuss the role of multiple factors in facial action production and interpretation.