Mother and infant smiling exchanges during face-to-face interaction in infants with and without Down syndrome.

We examined social smiling in infants with and without Down syndrome, aged from 3.2 to 13.6 months old. They were videotaped during an episode of spontaneous face-to-face interaction and a subsequent mother's still-face situation. Results indicated that infants smiled longer in the spontaneous face-to-face episode than in the still-face episode, even though this result was only significant in typically developing infants. Typically developing infants also smiled for a longer period than Down's syndrome infants during the spontaneous interaction episode. Moreover, infant's smile preceded the onset of the mother's smile, but in 6.2- to 13.6-month-old typically developing infants, the probability of mothers smiling before infants increased. These findings emphasize the possible existence of differences in the development of facial expression from signs to social symbols between infants with and without Down syndrome.