Quotes from Erik Homburger Erikson

An infant of two or three months will smile at even half a painted dummy face, if that half of the face is fully represented and has at least two clearly defined points or circles for eyes; more the infant does not need, but he will not smile for less. The infant's instinctive smile seems to have exactly that purpose which is its crowning effect, namely, that the adult feels recognized, and in return expresses recognition in the form of loving and providing.