Lateralization of the expression of facial emotion in humans

Though superficially symmetrical, the human face expresses emotion asymmetrically. Darwin (1872) first noted this phenomenon, conceding to being at a loss to explain why expressions such as smiling and sneering defiance were predominantly one-sided. Emotion lateralization offers a plausible account. Because the lower two-thirds of the face is contralaterally controlled, the emotion-dominant right hemisphere innervates the lower left hemiface, resulting in more intense expressions. Thus whether smiling or sneering, humans show stronger emotion on the left side of the face. This chapter reviews research examining asymmetries in the expression of facial emotion in humans, commencing with discussion of the right hemisphere's dominance for emotion processing. The right hemisphere's emotion-processing superiority results in hemifacial asymmetries in expressivity: the left hemiface is anatomically more expressive, moving more and earlier than the right hemiface. Not surprisingly then, viewers are sensitive to the left cheek's greater physiognomic expressivity, perceiving the left hemiface as more expressive than the right, even when digitally reversed. Critically, human behavior implies an intuitive awareness that the left cheek is more emotionally expressive, influencing behaviors including cradling infants and posing for photographs. Thus despite the absence of conscious awareness, when conveying emotion we intuitively favor the more expressive left cheek.