A three-month-old infant is lying in a crib. Slowly, the infant turns its head and eyes to each side as its gaze moves around the laboratory walls. The caregiver approaches the infant and leans over the crib in a face-to-face posture. The infant's gaze shifts across the caretaker's face and then settles on the caregiver's eyes. As the infant stares, its gross body movements cease. The infant's eyes brighten. The arms, legs and trunk produce a slight twitch and without even moving its mouth, the infant utters a short and soft vocalization. Hearing the infant's vocalization, the caregiver immediately raises her eyebrows, opens her mouth widely, and with one full nod of her head, inspires audibly. Then, the caregiver says the name of the infant. In response, the infant, still staring intently at the eyes of its caregiver and without moving its body, utters another sound. This time however, the infant's mouth is open and moving, the duration of the vocalization is considerably longer, and in sounds from the infant's mouth. In response, the caregiver asks, “Are you talking to me?”

In chapter 2, we presented evidence that infants develop the ability to participate in vocal interaction with adults before they are three months of age. Vocal communication between infants and adults is a synergistic social system. Therefore, my purpose in chapter 3 is to explore the effects of this system on the early development of vocal production in infants.