The presence of a sex difference in mutual eye-to-eye contact in dyadic interaction is well documented from late infancy through adulthood with females making more mutual eye contact than males. Only one study (Hittelman & Dickes, 1979) found evidence for this behavior pattern in newborns, but no research has been done to follow-up these findings. Systematic examination of the development of sex differences in mutual gaze behavior can aid in unraveling the differential effects of biological and social influences on the development of gendered social behavior.This project was a longitudinal, within participants replication and extension of Hittelman and Dickes study: Seventy neonates (32 female, 38 male) age 13--112 hours postpartum and their parents participated in the Time 1 data collection, and 23 (9 female, 14 male) infants and their parents were seen a second time at 13--18 weeks postpartum (Time 2). Mutual gaze between the infant and two interacters (1 female, 1 male) was measured, and parents completed the Parental Sex-Typing of Newborns (Paston) Rating Scale to measure their sex-typed perceptions of newborns and young infants.Results indicated: (a) No empirical evidence for sex differences at Time 1; (b) Strong evidence for sex differences in mutual gaze behavior at Time 2 indicating development of this sex-typed pattern in early infancy; (c) The emergence of sex differences in mutual gaze behavior from Time 1 to Time 2 is entirely accounted for by a radical change in female infants' gaze behavior; and (d) Empirical evidence linking mothers' sex-typed beliefs about their infants and infants' sex-typed gaze behavior.Results are discussed within the theoretical contexts of the social learning and biological perspectives. This study demonstrates that infants' sex-typed behavior and mothers' gender-typed perceptions begin early in life. It is concluded that sex differences in mutual gaze behavior are a complex interplay of biological or social forces acting in concert. Subsequent research in this area should focus on the specific forces involved in bringing sex differences in mutual gaze behavior to fruition