This research investigated the effects of age, sex, male vs female voice, and unfamiliar vs familiar voice upon the infants body movements. The sample consisted of 40 middle-class infants, balanced according to sex, at four age levels -- 3, 6, 9, 12 months. Four tape-recorded stimulus conditions -- mother's voice, father's voice, female stranger’s voice, and male stranger's voice -- were presented to each infant. The body movements of the infants were recorded at one frame per second by an 8mm. camera. Analyses of variance were performed to determine any differences among the four stimulus conditions and control periods. The 12-month-old infants showed more activity to the father's voice than to the male stranger's voice or to the mother's voice. These subjects also showed more activity to the female stranger's voice than to the male stranger's voice. Female subjects were more active than male subjects to the unfamiliar voices on Trial 1. On Trial 2 females showed greater activity than males to the familiar voice. Females exhibited greater activity than males overall. Males showed greater activity than females to the cessation of the female voice and to the initiation of their father's voice. Results were discussed in terms of the infant's development as affected by sex of the infant and the cross-over effect found by Friedlander (1970). / Master of Science