This study presents data on changes from 3 to 6 months in the type and frequency of infant facial expression. 60 mother-infant dyads were videotaped during play and reunion following a brief separation. Mothers' and infants' facial expressions were coded using the Max muscular components method. The mothers' verbal responses to infant expressiveness were also analyzed. Infants at both ages display a wide range of expressions and a high rate of change. Mothers respond contingently to 25% of infant changes; patterns of contingent responding varied slightly as a function of infant age and sex. Mothers show more contingent responding to older sons' smiles (vs. daughters' smiles) and follow sons' (vs. daughters') expressions with imitative expressions of their own. The only expressive difference between boys and girls at this age is that girls show more frequent interest expressions. Age-related changes included an attenuation of negative affect and a slower lability of expression change for older infants. The mothers' part in these age-related changes is revealed in the following results; Maternal expressions are limited to positive emotions, especially toward younger infants; mothers show less nonverbal and verbal acknowledgment of older infant expression change and do not acknowledge certain infant negative expressions. Finally, we report mother-infant dyadic similarities in expressiveness, including particular expression types and preferential use of the brow or mouth region in expressiveness. These results indicate that socialization of affect expression is occurring during early infancy and that the infants' expressiveness is becoming appropriate according to cultural, gender, and familial demands well before the first birthday.
