Changing determinants of crying termination in 6- to 12-week-old human infants

Developmental change in human infant crying termination was studied in 60 infants who were 6, 9, and 12 weeks of age. They qualified by spontaneously crying for at least 30 s during the study's first minute. Infants then received either 0.8 ml sucrose (12% w/v) by syringe or sucked a pacifier dipped in the 12% solution for a total of 3.5 min. Crying was substantially reduced by pacifier-sucking at all three ages tested. Sucrose too was effective at 6 and 9 weeks of age but did not reduce crying in 12-week-olds. These data suggest different schedules in the decline of sweet taste and pacifier sucking as agents of crying termination. These differential rates argue for multiple systems that govern crying reduction and argue against a single central target that changes ontogenetically in sensitivity and control. Whether the continuation of sucking effectiveness is sustained by infant sucking experience or reflects a slower rate of decline cannot be determined at present.