Facial expression form and function.

From an evolutionary perspective, facial expressions would serve adaptive functions that promote genetic fitness. While many ideas have been proposed,1 the specific adaptive functions of expressing emotion on the face have largely remained untested since Darwin proposed a set of expressive functional principles over 130 years ago.2 Recently, we showed that expressions of fear and disgust alter the biomechanical properties of the face, such that fear increases while disgust decreases sensory exposure.3 Additional vector flow analyses presented here reveal that anger and surprise expressions may similarly be shaped by sensory contraction and expansion action tendencies. An examination of the temporal dynamics of sensory modulation may reveal a general principle shaping expressive form rather than a specific adaptation shaping fear and disgust. Furthermore, if sensory modulation is a general principle, this function should be present across species rather than only in humans. Although facial morphology differs across species, detailed examination of sensory intake in different species may reveal the origins of facial expressions inherited by humans.