Infants are interested in human faces shortly after birth, and they take huge steps in their abilities to interact with people over their first few months. Between 4 and 7 months, features of babies’ personalities develop and are noticeable even to those meeting a child for the first time. At this age, infants will smile, laugh, and babble with their caregivers for minutes on end. While babies this age quickly grow bored of even the most engaging toy, most continue to enjoy attention from caregivers.
Shelov, S. P. (Editor-in-Chief). (2004). Caring for Your Baby and Young Child: Birth to Age 5. The American Association of Pediatrics. Revised edition. New York: Bantam Books.