Form and function in facial expressions of emotion
Facial expressions are instrumental in communicating one's internal emotional state and in regulating social interactions. However, facial expressions not only 'express' emotions but may also have adaptive functions for the sender beyond their established signal value. Over 130 years ago, Darwin hypothesized that emotional expressions originate in a less appreciated functional role, to modify preparedness for perception and action. We examined whether the origin of facial configurations associated with different emotional states may serve to alter the action of sensory systems whose receptors reside on the face in the service of regulating sensory input. We present evidence for this thesis through a convergence of methods, including visual statistical modeling of expression appearance, visual and olfactory psychophysics, eye movement recording, surface electrical potentials, and structural MRI. Our results provide evidence for two of Darwin’s evolutionarily derived principles of emotional expressive behavior: 1) the "principle of serviceable associated habits" - whereby specific facial expressions originate in patterns of movement serving adaptive functions for the sender, and 2) the "principle of antithesis" - whereby emotions with opposite functions are opposites in physical configuration. These convergent sources of evidence demonstrate that facial expressions are not arbitrarily shaped social signals, but rather originate as sensory adaptations, specifically configured to alter the biomechanical properties of the face.
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