Differential gazing and vocal response to mother and stranger of full-term and preterm infants across age.

Full-term infants at both 2 and 3 months and preterm ones at 3 months from due date vocalized more to mother than to a stranger and gazed more at a stranger than the mother. Preterm infants at 3 months from birth, but 6 weeks from due date, did not show differential responding by any mode, indicating the importance of maturational rather than environmental factors in these behaviors.