The development of smiling in the first months of life is an important indicator of an infant's level of social and cognitive functioning. Babies are born with a smiling reflex that is unrelated to any social impulse, a conclusion confirmed by the fact their earliest smiles, appearing within days of birth, occur almost solely while they are asleep. Researchers have described these early smiles as simple reactions to fluctuations in arousal: the relaxation produced when arousal reaches a modest threshold level and then declines produces a slight smile. If the arousal is too strong, relaxation doesn't occur soon enough to produce this smile. A physiological basis has been found for the contrast between this reflexive smiling and the social smiling that comes later: newborn smiles are associated with the lower brain regions, while social smiles involve the higher mental activities of the cerebral cortex. Newborn smiles (also called "sleep smiles") have been observed in infants whose cerebral cortexes are largely undeveloped or even missing entirely due to congenital abnormalities.
Social smiling, which occurs in response to the behavior of another person, usually begins between the ages of six and eight weeks. Initially, it is a response to pleasurable interaction with others in general rather than to a particular person. Although infants at this age do recognize their mothers, this recognition is not reflected in their smiles—they will smile as readily at the faces of others, including strangers. They will even smile at unpleasant-looking or unresponsive faces. According to child development experts, the pleasure shown by social smiling at this stage comes from an infant's satisfaction at having made sense of the world by recognizing a familiar object (a toy or mobile, or the human face in general, rather than a particular face), a phenomenon that has been termed recognitory assimilation. Laughter quickly follows the initiation of social smiling, often between the ages of two and three months. At this stage, babies will laugh at many of the same things that make them smile, with the laugh representing an intensification of the pleasure they feel. Social smiling in response to particular faces occurs at four or five months of age, when infants stop smiling at strangers and reserve their smiles for their mothers and other familiar persons.
Even before true social smiling begins, infants' smiles serve a social purpose by making their parents more attentive and affectionate toward them, which in turn helps the infants feel loved and secure. This pattern of reciprocal response helps lay the foundation for normal social development and parent-child bonding. Another way in which smiling holds a significance that extends beyond its underlying impulses is that through experience infants come to regard it as a way of manipulating their environment. Babies whose smiles elicit favorable responses, such as being picked up or being smiled at in return, learn that they can have an effect on their environment, and they are encouraged to smile even more. When smiling elicits no response, infants feel powerless and become depressed and unresponsive.
Bornstein, Marc H. Development in Infancy: An Introduction. New York: McGraw-Hill, 1992.
Brazelton, T. Berry, and Bertrand G. Cramer. The Earliest Relationship: Parents, Infants, and the Drama of Early Attachment. Reading, MA: Addison-Wesley, 1990.
Damon, William. Social and Personality Development: Infancy through Adolescence. New York: W.W. Norton, 1983.