smiling

A smile is a facial expression produced by upturning the corners of the mouth. The primary muscles are the zygomaticus major, which lifts the mouth, and the orbicularis oculi, which can raise the cheeks and create crow's feet in a genuine smile (often called a Duchenne smile). Non-Duchenne smiles involve the mouth alone.

Smiles function as social signals of positive affect, friendliness, and cooperative intent. They can influence others' impressions of a person, including trustworthiness and approachability. Some smiles are voluntary or polite masks rather than expressions of sincere emotion.

Infants produce reflexive smiles at birth; social smiles emerge around six to eight weeks as social interaction develops. Adults infer feelings and intent from smiles, though interpretation varies with context and culture.

Cultural norms shape smiling; display rules determine when and how much to smile in different settings. Consequently, smiling frequency and style differ across cultures and situations, and misinterpretations can arise from context differences.

Smiling is associated with psychological and physiological benefits, including mood enhancement for the smiler and better social bonding. It can reduce perceived stress and influence others’ behavior, though effects are often bidirectional and context-dependent.

Evolutionarily, smiling likely supports cooperation and social cohesion. Similar affiliative facial expressions occur in other primates, underscoring smiles as a broadly social tool.