Smiling of male and female infants to mother vs stranger at 2 and 3 months of age.

The study examined the recognition-assimilation hypothesis of infant smiling. The subjects were 64 (32 female and 32 male) full-term infants who were visited at their home at ages two and three months by a female experimenter. The mother and then the female stranger each interacted face-to-face with the infants for 3 min. All interactions were videotaped and later independent coders recorded the cumulative amount of time that the infants smiled to the mother and to the stranger. The results show that, whereas female infants smiled significantly more to the stranger than to the mother at both ages, male infants smiled more to the mother only at two months. At three months, male infants smiled about the same length of time to both mother and stranger. The results support the recognition-assimilation hypothesis and suggest that females develop faster socially.