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...