[Infants' responses to the still-face situation at five and nine months: focusing on sex differences]

This study investigated developmental changes and sex differences in infants' responses to a female stranger during a face-to-face interaction, still-face, and reunion phase. Twenty-two infants (11 boys and 11 girls) were observed at five and nine months of age. At five months, the infants smiled less in the still-face phase, while at nine months the infants smiled less at each subsequent phase. Girls, but not boys, smiled more at nine months than five months. Girls' gazing toward the social partner's face decreased during the still-face and reunion phases. Girls gazed toward the social partner's face more than the boys, independent of the phases and age. The findings support the view the effect of a still-face on infants is robust at both five and nine months and for both sexes. But the effect remained in the reunion phase only for the nine-month-olds. These findings suggest that girls have more interest in their social partner than boys.