Infants' expressive behaviors to mothers and unfamiliar partners during face-to-face interactions from 4 to 10 months.

Changes in the organization of infant looking, facial expressions, and vocalizations were examined over age (4, 7, and 10 months) and with different social partners. Although infants at all ages accompanied smiling with looking at both mothers and unfamiliar partners, 7- and 10-month infants accompanied vocalization with looking only when they were with mothers. Seven- and 10-month-olds vocalized with unfamiliar partners only when they were smiling at the same time. When mothers stopped talking, infants reduced smiling significantly at all ages, yet vocalized more at 10 months. In the second half of the first year, there are fundamental changes in the coordination of infant expressive behaviors that reveal a keen attunement to variations in maternal behavior and the familiarity of social partners.