The smile is the most easily recognised facial expression at a distance in human interactions. It is also an easier expression to make than most others. Other facial expressions denoting emotion – such as fear, anger or distress – require up to four muscles. The smile needs only a single muscle to produce: the zygomaticus major at the corner of the mouth (though a simultaneous twitching of the eyelid’s orbicularis oculi muscle is required for a sincere and joyful smile). As well as being easy to make and to recognise, the smile is also highly versatile. It may denote sensory pleasure and delight, gaiety and amusement, satisfaction, contentment, affection, seduction, relief, stress, nervousness, annoyance, anger, shame, aggression, fear and contempt. You name it, the smile does it.
