A longitudinal investigation of the acoustic properties of infant-directed speech from 6 to 18 months

Abstract

Caregivers often modulate their speech when interacting with infants, adapting a register that has been suggested to have attentional, affective, and didactic purposes. The present preregistered study examined the longitudinal trajectories of a diverse range of acoustic features of infant-directed speech (IDS), and compared these to adult-directed speech (ADS), in Norwegian parents of 6–18- month-old infants. Sixty-nine families participated. Throughout five lab visits across one year, par- ents were recorded reading a picture-book to their infant (IDS), and to an experimenter (ADS). The book was designed to tightly control for the linguistic content and context of speech between partic- ipants, timepoints, and registers. Analyses of a total of 54,594 vowels and 22,958 phrases revealed, first, an overall effect of register: Parents used higher pitch, wider pitch range, slower articulation rate, longer vowel duration, and more variable and less distinct vowels in IDS than in ADS. Second, significant register-by-age interactions indicated that parents’ IDS, compared to their ADS, featured wider pitch range, larger vowel space areas, and shorter vowel duration in older as compared to younger infants, while pitch, articulation rate, and vowel variability and distinctiveness remained relatively stable with age. Results are discussed in the context of the proposed functions of IDS.